Oh, I know it’s everywhere, but I just associate it with California for some reason. Maybe it’s only me.
ETA2: Oh, not to keep you in suspense, but the newer meaning means containing guacamole or avocado. Now, I didn’t grow up in the early have of the 20th Century, but I just picked up the “California burger” thing somehow, anyway. Maybe from just reading about hamburgers in general.
OK, I can see that. But I wasn’t around in the earlier half of the 20th century. 
Fresh tomatoes are a blunder; they simply make the burger sloppier than it should be, and difficult to handle. Lettuce adds nothing to the experience.
Raw onion and dill pickled slices, white American cheese, yellow mustard and a schmear of mayo to velvetize and add tang.
Congratulations! This is our ONE HUNDRETH “How to fix your hamburger properly” thread!
When I started making burgers in my panini press with the flat plates it almost changes my life. 1/4" thick & really crispy.
Like I said, neither was I. Born in '75. But I somehow picked it up along the way.
Mayo??!?
Get a rope.
Subtlety. Subtlety. This is a “how to fix MY hamburger properly” thread. Big difference. It’s a kinder and gentler time.
Grill your own - that’s the route to good burgers.
There’s only one place I’d rather eat a burger at than grill my own. Duffy’s Tavern, Anna Maria Island, FL. Their brie and caramelized onion burger is amazing.
USA Today rated them one of the top 10 burger places in the country. That’s how good a place has to be, to be better than what I can grill for myself.
Oh, and being a snob about what one should or shouldn’t put on one’s burger? Better than the same bit of snobbery with respect to hot dogs, I’ll grant you. BUT this is still ground beef we’re talking about, not filet mignon.
Honestly, I’ll take the ground beef four times out of five over the filet. Don’t get snobby yourself about meat, mister! 
I have! Thought I’d posted there, actually, but apparently not.
I like loading my hamburger up wth all kinds of stuff.
Sometimes I like it spicy, sometimes cheesy, sometimes meaty, I’m not picky.
My only rule is that I will not abide ketchup AND a tomato. It must be one or the other.
Perfect burger starts with steak. Trim off all visible fat, then grind the steak. Make a nice 1/4 to 1/2 pound patty and sous vide rare. Torch the exterior lightly. Serve on a bun, topping the burger with either a fried egg or a thick slice of avocado. Various condiments on the table.
OK, can we agree that avocado, fried eggs and fucking beetroot have no place on a burger?
Nope. Not a fan of avocado, but eggs are fantastic if you like them runny (and I know you don’t, otherwise I would agree it’s a waste), and beetroot is just another kind of pickle (and my Polish genes prohibit me from saying anything negative about beets.)
If this is something you worry about I don’t want your burger
See John “Best Food Wirter in America” Thorne’s transcendant hamburger essay “Burger Heaven.” Mayo is de rigeur.
Unless you don’t like mayonnaise on your hamburger. Then you can leave it off, no skin off my butt.
Hate mayo on a burger. But then, I love Thousand Island on one, so my opinion is highly suspect on the matter. 
If I ever find the guy that started the whole “Brioche bun for our upscale burger” trend I’m gonna kick him in the gahoolies. Brioche can’t stand up to either the condiments or the natural juices, so the bun disintegrates before you get 5 bites in.
Wait, what? I used to hate brioche buns on burgers, precisely because they were too rich and sturdy for a burger. Now the ones I see being sold commercially seem to be halfway between brioche and a normal hamburger bun, and stand up to a burger better than a bog-standard white bun. Now, I’m a fan of my white hamburger buns with sesame seeds (Martin’s, especially), but potato rolls are also where it’s at (Martin’s, as well.)