Bring a large pot of salted water to a boil. Add the cauliflower florets and blanch for 4-5 minutes until tender but firm. Drain and rinse under cold water.

In a medium saucepan, melt 3 tablespoons of butter over medium heat. Add minced garlic and sauté for about 1 minute until fragrant. Pour in 1 cup of heavy cream and bring to a gentle simmer. Whisk in 1.5 cups of grated Gruyere until smooth, seasoning with nutmeg, salt, and pepper.

Grease a baking dish with butter or cooking spray. Layer the blanched cauliflower evenly in the dish and pour the cheese sauce over it, ensuring all pieces are coated.

In a small bowl, combine 1 cup of panko, 0.5 cup parmesan, and chopped thyme. Melt 2 tablespoons of butter and drizzle over the mixture, stirring until combined.

Preheat oven to 375°F (190°C). Bake the dish for 20-25 minutes until bubbling and golden brown. Broil for an extra 1-2 minutes for a crispy top if desired.