Heat butter (or vegetable oil) over medium heat, add thinly sliced onions and saute until brown and caramelized (about 5 minutes) stirring as you go. Drain.
Place cauliflower in a microwaveable baking dish, add ½" water and cover with plastic wrap. Microwave on high for 5 to 7 minutes or until cauliflower is tender-crisp. Remove from water and cut into small chunks.
Do the same with broccoli, drain, cut into bite-size pieces.
In a medium bowl combine bacon, onion, mayonnaise, sour cream, 1 cup shredded cheese, cauliflower and broccoli. Mix well.
Pour mixture into 9"x13" (or round) baking dish. Top with remaining cheese.
Bake 20 minutes or until cheese is melted and slightly browned.
Notes
* Cauliflower and broccoli should equal about 6 to 7 cups. If you have more or less just adjust other ingredients up or down. Onions are optional. Sliced mushrooms or sliced jalapeno peppers would be a good addition.
Recipe by Pray Cook Blog at https://praycookblog.com/loaded-broccoli-and-cauliflower/