Steam cauliflower florets in a large pot filled with 1-inch of water until tender, about 5 minutes.
While cauliflower steams, cream together 3/4 of the cheddar cheese, 3/4 of the Monterey Jack, cream cheese, and heavy cream. Stir in sliced green onions, chopped bacon, and garlic. Set aside.
Drain any liquid from steamed cauliflower and add the cauliflower to the cheese mixture. Stir cauliflower and cheese mixture together until completely combined.
Taste for seasoning, and add salt and pepper as necessary.
Pour cauliflower into a 2-quart casserole dish and sprinkle on remaining cheddar and Monterey Jack cheese.
Cover dish with foil and bake for 25 minutes. Remove foil and continue to bake until cheese is brown and bubbly.
Remove from oven and serve warm.
Video
Notes
If you want a finer texture to your casserole, give the cauliflower a few mashes with the potato masher after stirring it into the cream cheese mixture.