I have some Granny Smith apples that are so tart that they aren’t very enjoyable to eat.
Instead of throwing the out, I decided to see what they’d be good with.
I know they’re good when baked into stuffing for Thanksgiving. Otherwise 
I ended up slicing one up and putting the slices into a tuna fish sandwich. And know what? It was pretty damn good! The apple added a nice texture and the tart flavor worked well with the tuna.
Granny Smith is the ‘default’ variety for baking into an apple pie.
Pretty much any baked or cooked application will be good. You may need to tweak the amount of sugar, but Suburban Plankton is right that many recipes already assume you’re using tart apples.
Lots of people like cabbage and apples, though it’s not really my thing.
They go well in salads (just sliced up fresh), where a sweeter dressing might help offset the tartness. And I don’t just mean fruit salad, but a a regular green salad.
Peanut butter goes well with apple slices and most commercial peanut butters are pretty sweet to boot.
You mention tuna. One of my wife’s classic macaroni salads starts with a fairly standard recipe and adds apples, walnuts and tuna. Most people are :dubious: when they hear about it and then 
after they try it.
Easy Fruit Dumplings (aka Cinnamon Crack Buns)
Plenty sweet enough to use very tart apples.
I use Granny Smith apples for pretty much everything apple-y that I make. Last night I made a pork loin roast with apples and onions- that was good. Apple crisp. Apple pie. Apple-oatmeal cookies. The other day I made this Buttery Apple Cake with them and it was so good, I can’t wait to make it again. If you make it, make sure you butter or grease the pan really, REALLY good, though.
I did a salted caramel apple upside down cake a couple of years ago that was outstanding using Granny Smiths.
Apple crisp.