Cook the egg noodles following the package instructions until al dente, then drain and set aside. Preheat the oven to 400 degrees F. Coat a 9×13-inch casserole dish with nonstick cooking spray, then set aside.
In a medium-sized saucepan over medium heat, melt the butter. Stir in the 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.
In a large bowl, mix the cooked noodles, drained tuna, prepared sauce, remaining 1 cup of whole milk, frozen peas, and shredded cheddar.
Spread the noodle mixture in the prepared baking dish.
Sprinkle shredded cheddar atop the casserole. Then, in a small bowl, microwave the butter until melted, then stir in the bread crumbs. Spoon that atop the cheese on the casserole.
Bake the casserole until the top is golden brown, which should take about 15-20 minutes.
Allow the casserole to cool slightly before spooning out servings.