Preheat oven to 200°C (400°F).
Rub the cauliflower with olive oil, salt, and pepper.
Roast for 45–55 minutes until tender and golden on the edges.
In a skillet, heat butter and olive oil. Add mushrooms and sauté until golden and reduced.
Add garlic, then stir in flour to make a light roux.
Gradually whisk in warm milk and cream until smooth.
Stir in Gruyère, mustard, nutmeg, salt, and pepper until creamy and silky.
Place the roasted cauliflower on a plate and pour the hot mushroom–Gruyère sauce generously over the top.
Garnish with chopped parsley or thyme.
