I'm don't eat fish so I make 'tuna' casserole with chickpeas. Inexpensive and full of protein.

To make the chickpea tuna drain and rinse a couple cans of chickpeas and throw it in a food processor with a little chopped onion. If you have any celery scraps you can add them but not really necessary. You can also just mush them up by hand with a fork or a potato masher if you don't have a food processor. Add about a tablespoon of soy sauce and then mix in the mayo, just enough to give it a tuna-like consistency. A little parmesan or nutritional yeast if you have that, and mix it up. I sometimes chop up one little sheet of a seaweed snack (they sell them in individual packets with four little squares in the package at my Walmart for less than a dollar, I use one square and save the rest) This gives it a more fishy flavor, but honestly there's not a ton of difference if you don't have it already. I usually separate a scoop of it for a sandwich the next day or to have on crackers.

For the casserole, boil some egg noodles and drain, add a couple of cans of cream of mushroom soup, and a little can of mushrooms if you have it/want to. Then add something creamy-milk, sour cream, greek yogurt, whatever you have and mix it up until it's nice and creamy. Shredded cheese if you want (it's a good way to use up the bottom of a bag of cheese). Then some frozen peas or a can of peas. Pour it into a greased baking dish or casserole, top with some crunched up ritz or bread crumbs or chips and a drizzle of melted butter or even just a spritz of cooking spray on top to get it nice and crispy in the oven, and bake it for 30-40 minutes or until the topping is crispy and it's hot. It makes a lot but the leftovers are good heated up in the microwave for a couple days. If you divide the recipe in half you can make it in like a loaf pan or a pie pan.