Bring a large pot of salted water to a rolling boil and cook your egg noodles according to package directions until just al dente. Drain them well and set aside. Slightly undercooking them prevents mushiness since they'll continue cooking in the oven.

In a large mixing bowl, combine the cream of mushroom soup, sour cream, and about half a cup of milk. Whisk until smooth and creamy. This creates a luscious sauce that coats every noodle perfectly.

Heat a tablespoon of butter in a skillet over medium heat. Add your diced onion and cook until softened and translucent, about five minutes. Stir in the minced garlic and cook for another minute until fragrant. These aromatics build incredible flavor depth.

Drain your canned tuna well and flake it with a fork. Add the tuna, cooked noodles, sautéed onions and garlic, frozen peas, and half of your shredded cheese to the bowl with your creamy sauce. Fold everything together gently until evenly combined. Season with salt, pepper, and paprika to taste.

Transfer the mixture to a greased nine-by-thirteen-inch baking dish, spreading it into an even layer. Sprinkle the remaining cheese over the top. In a small bowl, mix your breadcrumbs with melted butter until the crumbs are evenly coated, then scatter them over the cheese layer.

Slide your casserole into a preheated oven at 375°F and bake for twenty-five to thirty minutes, until the sauce is bubbling around the edges and the breadcrumb topping turns golden brown and crispy. Let it rest for five minutes before serving to allow the sauce to thicken slightly.