Preheat oven to 375-degree F and grease a medium-sized casserole dish with butter. Chop up cauliflower head into smaller bushels. Cook down cauliflower for 3 minutes in a large pot of boiling, salted water. Transfer cauliflower with a slotted spoon to a bowl of ice water to cool. Drain and lay cauliflower on a baking sheet lined with paper towels to completely dry.
Next, we will prepare our cheesy sauce, also known as preparing a “roux!” In a large skillet over medium heat, melt butter. Add garlic and cook for 1 minute until fragrant. 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 for 1 minute until slightly thickened.
Turn off heat and add 1 cup Gruyère, Parmesan, nutmeg and thyme. Stir until cheese has melted, then season with salt and pepper.
Now it’s time to layer our ingredients. Add about half the cauliflower to your greased casserole dish, then pour in half of the cream mixture. Repeat with remaining cauliflower and cream, then top with 1 cup of cheddar cheese.
Bake for 25 minutes or until bubbly and golden on top.
Garnish with parsley and serve warm.