Cook Noodles: Cook egg noodles following package instructions until al dente, then drain and set aside.
Preheat Oven: Preheat oven to 400 degrees F. Coat a 9x13-inch baking dish with nonstick cooking spray, then set aside.
Make Sauce: In a medium-sized saucepan over medium heat, melt butter (see note below). Stir in flour and cook for 1 minute. Gradually add 2 cups of whole milk, stirring constantly. Cook until thickened into a sauce, which may take upwards of 10-15 minutes, stirring occasionally. Season with salt and pepper.
Combine Casserole Ingredients: In a large bowl, mix cooked noodles, drained tuna, prepared sauce, remaining 1 cup of whole milk, frozen peas, and shredded cheddar.
Transfer Casserole Mixture To Baking Dish: Spread noodle mixture in prepared baking dish.
Prepare Topping: Sprinkle shredded cheddar atop casserole. Then, in a small bowl, microwave butter until melted, then stir in bread crumbs. Spoon that atop the cheese on the casserole.
Bake: Bake casserole until the top is golden brown, about 15-20 minutes.
Serve: Allow casserole to cool slightly before spooning out servings.