*Add desired amount of salt to the sauce AFTER you've added the cheese and the cheese has melted. Blue cheese, for example, is quite salty so you may not need to add.
Preheat oven to 350 degrees F. Lightly grease or spray a 2-quart baking dish.
Shred or crumble any cheese you are using.
Partially cook cauliflower by placing frozen florets in a deep saucepan and covering with water. You can add a tsp of salt if you wish. Cover and bring to a boil, reduce heat and simmer for 3-5 minutes. Drain and place in prepared baking dish.
Melt the 3 tbs of butter in a medium saucepan over medium-low heat. Add the flour and blend (this is a roux). When roux begins to bubble, slowly add the milk, while constantly stirring, to prevent lumps. Cook, stirring constantly, until mixture thickens and begins to bubble. Remove from heat and add whichever cheese you are using. Stir until cheese is melted (yes, you can use more or less cheese depending on your taste). Correct any seasoning at this point.
Pour sauce over cauliflower in baking dish.
Mix topping ingredients and distribute over top of casserole.
Bake for 30-40 minutes or until completely heated through and sauce bubbles.