Preheat your oven to 375°F (190°C) and grease a 2-quart baking dish with butter or cooking spray.

Bring a large pot of salted water to a boil. Add cauliflower florets and cook for 5-7 minutes, until tender. Drain and transfer to the prepared baking dish.

In a saucepan over medium heat, melt butter and sauté garlic for 1 minute until fragrant. Add flour and whisk to form a roux, cooking for 2 minutes until golden.

Gradually whisk in milk, ensuring no lumps remain. Cook while whisking until thickened, about 5 minutes. Remove from heat and stir in Gruyère cheese, ¼ cup Parmesan cheese, salt, pepper, and nutmeg until smooth.

Pour the cheese sauce evenly over the cauliflower in the baking dish. Top with breadcrumbs and the remaining Parmesan cheese.

Bake for 25-30 minutes, until golden brown and bubbling. Garnish with fresh parsley before serving.