I also keep saying I’m not a trained chef. 
No real reason: I just don’t like the texture of leftover mashed, and there are seldom any left to deal with. I also didn’t realize that I’ve been beating this dead horse. I’ll stop now.
If only the OP had thought of that!
Grill and chop into small pieces a couple of rashers of bacon, mix in a cup or two of cottage cheese and a small-ish onion chopped fine, bake until warmed through and serve up with baked beans.
Use it as frosting on a meat cake
I often make a fried goulash out of potato leftovers (and whatever else is in the fridge) - potatoes, diced ham, chopped onions, garlic if you like, maybe some broccoli, definitely cubed cheddar cheese, maybe an egg or two, and salt and pepper - and just fry it all up until parts are golden brown and all is heated up. It’s pretty darned tasty, if I do say so myself.
Oh. Well, that part I’ve been missing. FWIW, you do have a very authoritative style, that conveys a sense of authentic mastery of your subject.
That’s okay. I’m just glad to know that I’m not slowly poisoning my family with potassium hydroxide, formed by the mixture of potatoes, butter, milk, NaCl, and the small amount of capsacin found in black pepper when exposed to several hours of cold.
Here’s what the US Senate does with 'em:
Personally I fry them up on a cold morning, seasoned with butter & salt or some nice hot salsa. Got to get that crunchy coating on them so I break out the cast iron pan. Enjoy with a mug of tea or cocoa while doing the crossword puzzle and playing with the cat.
If I happen to have leftover steak handy all the better, steak & potatoes = yum.
Here’s one variation of the many recipes in that search:
MASHED POTATO COOKIES
1 1/2 c. flour
1/2 tsp. salt
1/2 tsp. baking soda
1/2 c. butter
1 c. brown sugar
1 tsp. vanilla
1 egg
3 oz. unsweetened chocolate, melted, cooled
1/2 c. mashed potato
3/4 c. buttermilk
Cream butter and sugar until fluffy. Add egg and vanilla. Mix. Add chocolate and mashed potato and mix well. Add dry ingredients alternately with buttermilk, mixing just enough after each addition to combine ingredients. Drop by spoonfuls onto ungreased cookie sheets. Bake at 375 degrees for 12 to 13 minutes. Frost immediately with chocolate icing.
Ah, you added garlic, I see… a bit too late. Personally, I like chocolate and garlic together.
You could use (some of) them in Tourtière.
We also fry them in a pan with some butter and pepper. They’re yummy reheated that way for supper, and are good with breakfast instead of hashbrowns.
You probably don’t want even more potatoes on top of those, but really finely mashed potatoes can be substituted for some of the mayo in the dressing for potato salad* and you won’t be able to tell. You just need more mustard or relish or whatever it is you use to flavor the sauce.
*The gooey American kind, not the bacon-y kind.
Kind of riffing off Leiko’s Idea, you could incorporate and include them in a Deviled mixture along with a dressing. For example add a 1/2 cup or so to your favorite deviled egg recipe.
Maybe construct a spinach, tomato, egg, tuna, and black olive salad… A Nicioise. Make a hot bacon, sweet and sour, mashed potato vinaigrette for the nicioise. Sort of a riff on the classic American Spinach Salad with warm bacon vinagrette. Wilted Spinach Nicoise.
This looks like a great recipe for a Potato Souffle from the food network… Your leftover potatoes with blue cheese in a light and fluffy delivery.
Use them as a thickener for clam chowder.
Cheesy Clouds!
Mix grated cheese of your choice into the 'taters. I like gruyere, but sharp cheddar is good, too.
Roll the cheese / 'tater mixture into firm balls, slightly larger than a golf ball.
Coat with beaten egg, then coat with bread crumbs (Panko works best).
Bake at 350 for 15- 20 minutes.
YUM!!
Mashed potatoes with meatballs, brown sauce (brown gravy) and lingonberries. You can usually find decent Swedish meatballs and lingonberrie jam at your local Ikea.
Cooking is a hobby and an off-and-on-again passion. I take a failed dish very personally, because what I do, I do very well. I also think I understand seasoning as well as or better than most. But every once in a great while a train wreck happens, like the soup I made from the prime rib bones. I took a wrong turn somewhere, and it ended up in the disposal. I’m still pouting over it.
Speaking authoritatively doesn’t always mean someone is right, merely highly opinionated. 
Well, you certainly talk a good game, and I hope it doesn’t sound insincere to tell you that I regard your cooking-related posts quite highly.
My favourite use for leftover mashed spud is Boston Bun ex-designz : Un blog de design mais pas que !
OK, it looks like we’ll be making a lot of mashed potatoes around here for the next few weeks… who knew there were so many uses for the humble mashed spud?
In case anyone is curious with what happened, I had a sudden flash of inspiration on the way home and ended up going with the potato pancake idea after all, but gave it an aloo mattar twist by stirring in a couple spoonfuls of curry paste, a sprinkle of cumin seed and a handful of frozen peas. The curry nicely cancelled out any possible hint of the dreaded leftover mashed potato flavour, and the whole crispy outside fluffy inside thing works very well with the flavour combo. A very successful experiment, if I do say so myself.
Use them to make a scale model of Devil’s Tower
LOL…a man after my own heart! 
I made Train Wreck Soup a month ago. It was so bad that I froze some so I can hate myself every time I open the freezer. I may let it go this week.