Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or butter.
In a large pot of boiling salted water, blanch the cauliflower florets for about 5 minutes until slightly tender. Drain and set aside.
In a large skillet over medium heat, add the olive oil. Sauté the sliced leeks for about 5 minutes until soft. Add the minced garlic, salt, pepper, and nutmeg, cooking for another minute until fragrant.
Pour in the heavy cream and bring to a simmer. Stir in the Gruyère cheese until melted and smooth. Remove from heat.
Combine the blanched cauliflower with the cheese sauce, mixing until the florets are well coated. Transfer the mixture to the prepared baking dish.
In a small bowl, combine the breadcrumbs with the melted butter. Sprinkle the breadcrumb mixture evenly over the cauliflower and cheese.
Bake in the preheated oven for 25-30 minutes, or until the top is golden brown and bubbly. Let it cool for 5 minutes before serving.
Garnish with chopped parsley before serving.