Preheat oven to 400. Grease a 13X9 inch baking dish with cooking spray.
Heat 1 tablespoon butter in a large skillet until melted. Add onion and garlic. Sauté 2-3 minutes. Add carrots, broccoli, and cauliflower. Saute 5 minutes. Place vegetables in a large bowl and set aside.
Add remaining butter to skillet and heat until melted. Add flour. Whisk until smooth. Stir in milk.
Bring to a simmer. Simmer 5-7 minutes. Season with salt and pepper. Add to vegetable mixture. Stir well. Add to baking dish. Top with both cheeses.
Bake at 400 25-30 minutes. Let stand 5 minutes before serving.
Notes
Variation: Ham and Garden GratinAdd 12 oz chopped uncured ham (no sugar listed in ingredients) with the vegetables. Bake as directed.