Preheat the oven to 350 Degrees F. Bring a large pot of water to boil. Boil macaroni until just tender, about 8 to 10 minutes.
While the pasta cooks, place the 3 Tbsp. of butter in a medium-sized pot set over medium heat. When melted, mix in flour and cook and stir 1 minute. Slowly, while whisking steadily, mix in ½ cup of the milk. Cook until the mixture becomes quite thick, and then slowly whisk in remaining milk. Bring the sauce just to a simmer, stirring frequently, and then remove from the heat. Mix in the ketchup and three-quarters of the cheese; season with salt and pepper.
Lightly butter an 8" x 8" baking dish. When the macaroni is cooked, drain well and mix it, the tuna and the peas into the sauce. Spoon the macaroni into the baking dish. Top with remaining cheese. Bake 20 to 25 minutes, or until light golden and the sauce is bubbling. Sprinkle servings of the macaroni, if desired, with chopped parsley.
Note: To warm the milk, place in a glass measuring cup or bowl, set in the microwave, and heat 1 to 2 minutes.