Preheat oven to 400°.
Cook pasta in boiling water 8 minutes; add broccoli. Cook 3 minutes or until pasta is done. Drain.
Heat 2 teaspoons oil in a Dutch oven over medium heat. Add garlic to pan; cook 45 seconds. Transfer garlic mixture to a small bowl; set aside.
Add remaining 1 teaspoon oil to pan; stir in onion. Cook 1 minute, stirring frequently. Remove from heat; stir in 3 cups milk. Lightly spoon flour into a dry measuring cup; level with a knife. Combine the remaining 3/4 cup milk and flour in a small bowl, stirring with a whisk. Add flour mixture to onion mixture. Return pan to medium-high heat, stirring constantly with a whisk; bring to a boil. Cook 1 minute or until thick, stirring constantly with a whisk. Remove from heat; stir in the cheeses, salt, black pepper, red pepper, and nutmeg. Add pasta mixture to milk mixture, tossing gently to coat. Pour mixture into a 13 x 9-inch baking dish coated with cooking spray.
Combine garlic mixture and bread in a food processor; pulse 15 times or until fine crumbs measure 1 cup. Sprinkle breadcrumb mixture over pasta mixture. Bake at 400° for 18 minutes or until the top is brown.