Cauliflower Au Gratin

Preheat oven to 375° and butter a medium casserole dish. In a large pot of boiling salted water, cook cauliflower for 3 minutes. Use a slotted spoon to transfer to a bowl of ice water to cool. Drain, then lay cauliflower out on a baking sheet lined with paper towels to let dry completely.  In a large skillet over medium heat, melt butter. Add garlic and cook until fragrant, about 1 minute. Whisk in flour and cook until flour is golden and starts to bubble, about 1 minute more. Add milk slowly and stir until mixture comes to a simmer. Boil until slightly thickened, about 1 minute. Turn off heat and add 1 cup Gruyère, Parmesan, and thyme. Stir until cheese has melted, then season with salt and pepper.  Add about half the cauliflower to prepared pan, then pour in half of the cream mixture. Repeat with remaining cauliflower and cream, then top with remaining 1 cup of Gruyère.  Bake until bubbly and golden on top, about 25 minutes. Garnish with parsley and serve warm.