Place broccoli in a 12- x 8- x 2-inch baking dish. Saute onion in butter; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Add Parmesan cheese, salt, mustard, pepper, paprika, and marjoram; stir until cheese melts. Spoon sauce over broccoli; bake at 375° for 15 minutes. Sprinkle Cheddar cheese over top; bake 5 minutes.