Cook spaghetti according to package directions, drain thoroughly, and et aside.
In a large heavy saucepan, melt butter or margarine over moderate heat; add green pepper and sauté about 2 minutes. Add mushrooms and continue sautéing for 2 minutes or just until mushrooms are tender but firm, stirring constantly. Stir in flour. Gradually add milk or milk and half-and-half, blending well. Cook, stirring constantly, over moderate heat until mixture is thickened and bubbly hot. Season with salt and white pepper to taste. Add cooked spaghetti, turkey, Swiss or other shredded cheese, and pimento. Adjust seasoning to taste.
Transfer pasta mixture to a shallow 1 to 1½ -quart casserole liberally sprayed with non-caloric vegetable cooking spray. Bake, uncovered, in a preheated moderate oven (350 degrees F.) for 20 minutes. Sprinkle top of casserole evenly with grated Parmesan or Romano cheese. Continue baking for an additional 7 to 8 minutes or until grated cheese is lightly browned and casserole bubbly hot. Garnish as desired and serve immediately.
Note: May use reduced-fat or skim milk, if desired; however, the flavor will not be as rich.
Note: If using chopped sweet red pepper, sauté sweet red pepper with the green pepper.
Note: For 8 servings, double the ingredient amounts and bake the casserole for about 35 minutes total.