Wash the cauliflower thoroughly and cut it into small florets, ensuring they are roughly the same size. Set aside.
Melt the unsalted butter in a large skillet over medium heat. Add the sliced leeks and sauté for 5 minutes until soft. Stir in the minced garlic and cook for an additional minute.
In a separate bowl, whisk together the heavy cream, shredded white cheddar cheese, Dijon mustard, salt, black pepper, and nutmeg until smooth and creamy.
Add the cauliflower florets to the skillet with the sautéed leeks. Pour the creamy cheese sauce over the mixture, stirring gently to coat. Transfer to a greased 2-quart baking dish. Combine breadcrumbs and grated Parmesan cheese, and sprinkle over the top.
Preheat the oven to 375°F (190°C). Bake for 25-30 minutes until the top is golden brown and the sauce is bubbling. Let cool for a few minutes before serving.