Preheat the oven to 200°C (400°F).

Toss the cauliflower florets with olive oil, sea salt, and black pepper on a large baking sheet.

Roast for 20 minutes until the edges are caramelized and the cauliflower is tender-crisp.

In a medium saucepan, melt the butter over medium heat, whisk in the flour, and cook for 2 minutes to form a blonde roux.

Gradually whisk in the milk and heavy cream, stirring continuously to prevent lumps, until the sauce thickens and coats the back of a spoon.

Remove the pan from the heat and stir in the mustard powder, nutmeg, 100g of the cheddar, and all of the Gruyère until smooth and melted.

Arrange the roasted cauliflower in a 2-quart baking dish and pour the cheese sauce evenly over the florets.

Top with the remaining 50g of cheddar and the Parmesan cheese.

Bake for 20-25 minutes until the topping is golden-brown, crisp, and bubbling.