Preheat oven to 425°F and lightly grease a 9x13-inch baking dish.
Toss cauliflower florets with 2 tablespoons melted butter, salt, and pepper, then spread on a baking sheet and roast for 15 minutes until slightly tender.
Meanwhile, in a saucepan over medium heat, melt remaining butter and sauté garlic for 1 minute until fragrant.
Add heavy cream, Dijon mustard, thyme, and remaining salt and pepper to the saucepan, bringing to a gentle simmer.
Remove from heat and stir in 1 cup cheddar cheese and half the Parmesan until melted and smooth.
Transfer roasted cauliflower to the prepared baking dish, pour cheese sauce over top, and sprinkle with remaining cheddar and Parmesan.
Bake for 20-25 minutes until bubbly and golden brown on top.