Preheat oven to 350°.
Heat oil in a large saucepan over medium-high heat. Add onion; sauté 3 minutes or until crisp-tender. Add bell pepper; sauté 3 minutes. Remove mixture from pan; set aside.
Cook pasta according to package directions, omitting salt and fat. Drain.
Combine milk and flour in a medium bowl, stirring with a whisk. Melt butter in saucepan over medium-high heat; add milk mixture, stirring with a whisk. Cook 3 minutes or until slightly thick, stirring constantly. Remove from heat; gradually add cheeses, stirring with a whisk until cheeses melt. Stir in salt, black pepper, nutmeg, and ground red pepper. Add onion mixture and pasta, stirring well to combine. Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray. Sprinkle breadcrumbs evenly over pasta mixture. Bake at 350° for 35 minutes or until bubbly. Garnish with parsley, if desired.