I love pickled Herring, especially in cream sauce. Definitely an “ethnic” food, though…
I’ve been looking for mutton, too - I’d hope it is cheaper than lamb, and the dogs would love it! (I’d love it, too, but I’m not sure I could get anyone else to eat with me…)
Still is. Look in the Mexican aisle or a Mexican mart. It’s pretty much required for Mexican wedding cake. I love to use it in couscous. I haven’t tried much more than that.
hijack: Rose water is a by-product of making Rose Absolute, or essential oil through a chemical extraction process. After several steps, each involving a more noxious solvent than the last, you get one small vial of pure essential oil of rose that’s rather poisonous (in quantity, anyway) and one bucket of vaguely rose-scented water that’s safe to drink. That stuff in the bucket is rose water. If you go through aromatherapy supply stores instead of groceries, you can also find lavender water, yarrow water and peppermint water, among others. All of them are safe to drink straight (my aromatherapy teacher drank nothing but rose water, and she smelled like a garden!) or use in foods.
Depends how you cook it. Ground or minced, as it’s usually found in tacos, you wouldn’t know it wasn’t ground beef. (And of course technically, it is ground beef!) Tongue is often sliced or cubed and used in soup or stew, where it’s cooked a long time, and like any beef in soup or stew, yes, it gets stringy but you don’t really care in that context. It can also be braised (cooked in a simmering liquid) whole, but most people are a little put-off by the bumpy taste buds. I trim that layer off before cooking, and it cooks up just like a tiny pot roast.
I wouldn’t eat it as steak, mind you. But cooked in liquid, it’s pretty much like any beef muscle. Just throw away the package before anyone gets home so they don’t see “tongue” on it and freak out on you! 
Me too. And if* I *can’t find it, I’m guessing there’s a reason - like USDA laws or something - that I can’t find it. I live on the North Side of Chicago, in the most ethnically diverse neighborhoods in the country, and no mutton. Not even in the buy-a-live-goat-or-chicken-or-rabbit-and-we’ll-kill-it-for-you-in-back butcher shops. Not even frozen in the butcher shop that carries rabbits and doves and whole octopus and hog’s heads and chicken feet and cow’s kidneys. It’s just not available.
Might have to find a wool farm and order one under the table. My guess is they’re sold off for animal feed, and prohibited for human consumption for some reason.
Green goddess dressing. I hadn’t seen it for years until I finally found a bottle of it recently.
I was talking about tamales made with cow head (AKA cabeza de res). (Although tamales can be made with any kind of meat & many vegetables.) Tamales are time consuming & often are only made during the holidays–when cow heads & fresh masa show up at the neighborhood grocery.
Menudo is a soup made from tripe. Or you can just cook up the tripe to make* tripitas*.
Just about anything might show up inside a taco.
Here in Texas, the Louisianans make spicy head cheese. They make pretty good boudin & dirty rice, too.
(And I’ll agree with the oxtail love in this thread.)
Rose water and rose syrup are also popular ingredients in Indian cuisine.
As far as gizzards go, I’ve been to Harold’s Chicken Shack here in Chicago and seen them sell out of gizzards at lunchtime.
The only thing I’ve ever had that was flavored with rose water was Turkish delight, which sounded so good in the Narnia books but really repulsed me.
I’d wondered about our lack of mutton myself. Prince Charles founded The Mutton Renaissance to help UK farmers.
Growing up, we never had lamb–apparently because older relatives had traumatic memories of institutional mutton. This New York Times article confirms my suspicion.
On my own, I discovered that lamb can be delightful. Apparently, there’s no mutton available because nobody wants it. Of course, how can you want it if you can’t find it?
You might check halal butchers.
Central Iowa.
As others have noted, mutton is now becoming available in the UK again, but it isn’t cheap! Like for like it may be a bit cheaper than lamb but it is very much a speciality product, normally organic and often from rare breeds, and it is way more expensive than lamb from the supermarket.
The biggest group to fall out of popular favour I can think of is offal in general. As a child I eat liver, kidneys, and heart but we never gave them to our kids and they look at you as if you were mad if you suggest anything like them.
Just knowing that I’m not alone in my love of gizzards has made this a better day. Thanks, all.
Ox-tails have gotten absurdly expensive. Last time I looked, they were going for north of six dollars a pound, but they’re regularly stocked at our Safeway, so somebody’s buying them. At that price, it’s not me.
Same goes for tongue - it’s there on a foam tray, just waiting to be bought.
Veal - whatever happened to veal? Ask any Italian, and they’ll rattle off a dozen or more ways their Mama made veal. Now, it’s scarce. I’m guessing it’s fallen out of favor as more people are realizing the unpleasant short life a calf faces if it’s destined to be butchered as veal.
Much to my partner’s dismay, I have no trouble finding limburger cheese at a market near my office.
Not to pile-on the fondue train, but, yeah. Not only is it still popular (you can get fondue pots everywhere, but it’s evolved. Try going to, say, a bridal show that doesn’t have a dozen companies with their chocolate fountains (which you can also buy everywhere). I have a chocolate fountain, **and ** a fondue pot, and I’ve actually done fondue parties.
You can get a family pack of chicken feet at the Piggly Wiggly in my town. This is the same store where you can go through the meat section and pick up all the parts you need to assemble your own pig.
Several people have metioned pig ears falling out of favor. Most of the ears I see are dog chew toys these days. Who wants to eat dog food? (would be the reason why they’re not eaten much anymore).
Almost embarrassing confession time:
We bought some of those Elegant Medley cat foods for our cat Orson to try to tempt him to eat and when I opened one that was white meat chicken with a cheese souffle (or something similar to that) I was tempted to taste it. It really smelled good!
But my husband was there, laughing at me, so I didn’t. 
Dried beef. Not jerky - the stuff we had periodically growing up was very thinly sliced, fairly bright red, somewhat moist, VERY salty, and sold in small flat packages - seems like I’d also seen it in jars but Mom got the plastic-wrapped flat package. Mom used to chop it up and heat it up in a can of cream of mushroom soup (condensed, diluted with milk) and serve it ladled over toast. I have not seen the meat in stores since growing up (admittedly, I haven’t looked for it).
Wasn’t until I was grown up that I learned another name for this dish: SOS (S— on a Shingle). Mom, obviously, never called it that (nor did Dad, who’d likely eaten a fair bit of it in the Merchant Marines).
I actually rather liked it - somehow the curl-your-intestines salt level overwhelmed the rotting-vegetation taste of the fungus in the soup. I’m not sure you could persuade me to eat it these days though.
Penzey’s sells a great Green Goddess dressing mix - it’s awesome for dipping red bell peppers into.
It’s still there. It’s good to buy for roll ups with cream cheese (for little appetizers).
My mom made creamed beef, too, but just with a roux instead of the soup. Over toast. Mmm.
Concerning Mutton… Owensboro, Kentucky and Western Kentucky is the modern home of Mutton in America. It’s the Barbecue of choice in Western Kentucky and they even have their own BBQ festival with a Mutton eating contest. They have a famous restaurant called the Moonlite BBQ Inn whose specialty is BBQ Mutton, and they sell their BBQ Mutton mail order. You could probably also give them a call or do some searching online and see if there are any Butchers or specialty purveyors in Owensboro who deliver fresh Mutton.