In a 15x10x1-inch baking pan combine crumbs, Parmesan cheese and the 1 Tbsp. butter. Bake 5 minutes in a preheated 425-degree oven. Remove from oven, stir, set aside.
Meanwhile, in a Dutch oven, cook spaghetti, with 1 Tbsp. salt added to water, according to package directions; add peas during last 3 minutes of pasta cooking time. Reserve 2 cups pasta cooking water; set aside. Drain pasta and peas. Keep warm.
Return 1 cup reserved pasta water to Dutch oven. Bring to boiling. Add the 2 Tbsp. butter and mustard. Add cheeses, a bit at a time, stirring after each addition until melted. Stir in more pasta water if needed. Add spaghetti mixture; toss. Sprinkle servings with crumb mixture and prosciutto.