1. Preheat oven to 375°.
2. Cook cauliflower and Brussels sprouts in boiling water 2 minutes; drain.
3. Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, milk, and next 4 ingredients (through nutmeg) in a bowl; stir well with a whisk.
4. Heat a large skillet over medium heat. Add bacon to pan; cook 3 minutes or until bacon begins to brown, stirring occasionally. Add onion and garlic; cook 5 minutes, stirring occasionally. Stir in milk mixture; bring to a simmer. Cook 5 minutes or until thick, stirring constantly. Remove from heat; stir in cauliflower and Brussels sprouts. Spoon vegetable mixture into an 11 x 7-inch broiler-safe ceramic baking dish coated with cooking spray. Cover dish with foil coated with cooking spray. Bake at 375° for 20 minutes or until bubbly. Remove from oven.
5. Preheat broiler to high.
6. Remove foil from dish. Combine cheese and panko; sprinkle evenly over vegetables. Broil 5 inches from heat for 4 minutes or until browned. Let stand 5 minutes before serving.