In a large bowl, stir together the cream of mushroom soup, sour cream, milk, salt, pepper, garlic powder, and Parmesan cheese. Gently fold in the frozen peas, mushrooms, tuna, and cheddar cheese. Set aside.
Spray a 2 quart baking dish with non-stick spray. Pour in the casserole ingredients; spread evenly. Finely crush Ritz crackers into small crumbs (put them in a baggie and use a rolling pin to crush them). Once crushed, sprinkle crumbs over the top of the casserole.
Bake (uncovered) at 350 degrees for 40 minutes. Remove pan to a wire rack. Let it cool for a few minutes before slicing (letting it cool a bit will let you make better cuts, and make it easier to dish up!) Serve, and enjoy!
Mom's Tuna Noodle Casserole https://www.thegratefulgirlcooks.com/moms-tuna-noodle-casserole/