Preheat the oven to 375 degrees F. Coat a 3 quart baking dish with cooking spray.
Melt the butter over medium heat in a large pan. Add the garlic and cook for 30 seconds.
Add the flour and cook for 1 minute, stirring constantly.
Gradually add in the milk, whisking constantly. Season with salt and pepper to taste, then bring the milk mixture to a simmer.
Cook for 4-5 minutes, stirring occasionally, or until thickened. Stir in the parmesan cheese and 1 ½ cups of cheddar cheese.
Add the cauliflower to the cheese sauce and toss to coat. Stir in the bacon and green onions.
Pour the cauliflower mixture into the prepared baking dish and top with remaining cup of cheddar cheese.
Bake for 25-30 minutes or until casserole is bubbly and cheese is melted. Let stand for 5 minutes, then serve, garnished with additional bacon and green onions as desired.
