Preheat oven to 400°.
Cook broccoli in boiling water 3 minutes or until crisp-tender; drain.
Lightly spoon flour into a dry measuring cup, and level with a knife. Place flour, salt, and dry mustard in a large, heavy saucepan over medium heat; gradually add the fat-free milk and chicken broth, stirring with a whisk until blended. Cook for 8 minutes or until it is thick, stirring constantly. Remove from heat, and add shredded cheddar cheese and 6 tablespoons of grated Parmesan cheese, stirring until melted. Stir in pimientos and pepper. Add broccoli, tossing to coat.
Spoon the broccoli mixture into a 13 x 9-inch baking dish coated with cooking spray.
Place garlic melba toast rounds in a food processor; pulse 10 times or until coarse crumbs measure 1 cup. Combine toast crumbs and the remaining 2 tablespoons Parmesan cheese, and sprinkle evenly over broccoli mixture. Spray top of casserole lightly with cooking spray. Bake at 400° for 15 minutes or until bubbly. Let stand for 5 minutes before serving.