Preheat oven to 400º. In a large pot of salted boiling water, cook cauliflower until tender, 8 minutes. Drain well.
Butter a large baking dish. Add half the cauliflower and pour over half the heavy cream. Dot with half the butter, sprinkle with half the garlic, cheddar, Parmesan, and thyme. Repeat with remaining ingredients and season with salt and pepper.
Bake until cheese is melty and deeply golden, 30 minutes.