Preheat oven to 400 degrees and generously butter an 8" square casserole dish.

Bring water to boil in a large pot over high heat and cook cauliflower for 6 minutes. Remove from heat, strain then place in a casserole dish.

Sauté onion and garlic in butter over medium heat until onion is translucent, about 2 minutes. Add flour, whisk to combine then cook for 1 minute. Slowly whisk in milk until flour is dissolved. Stir in cheeses until melted. Turn off heat and season with salt and pepper to taste.

Pour cheese sauce over cauliflower then bake 20-25 minutes or until hot and bubbly. Cool for at least 5 minutes before serving.