1. Place flour in a medium saucepan. Gradually add milk, stirring constantly with a whisk until smooth. Cook over medium-high heat 2 minutes or until mixture is bubbly and thickened, stirring constantly. Cook 1 additional minute, stirring constantly. Remove from heat. Add cheeses, mustard, salt, pepper, and thyme, stirring with a whisk until smooth. Keep warm.
2. Add water to a large saucepan to a depth of 1 inch; set a large vegetable steamer in pan. Bring water to a boil over medium-high heat. Add broccoli to steamer. Steam broccoli, covered, 4 minutes or until crisp-tender. Serve broccoli immediately with sauce.