Roasted Garlic Gruyère-Style Cheddar Mornay

Steps

1. Preheat the oven to 400°F (200°C).
2. Set the garlic head on a square of foil, drizzle the cut top with the olive oil, wrap it tightly and roast for 40 to 45 minutes, until the cloves are soft and golden. Let it cool for 5 minutes, then squeeze the cloves into a small bowl and mash them into a paste with a fork.
3. While the garlic roasts, shred the cheddar, mince the onion and warm the milk in a small pan or the microwave until steaming.
4. Melt the butter in a medium saucepan over medium-low heat. Add the onion and cook for 3 minutes, until soft but not browned.
5. Whisk in the flour and cook, whisking, for 2 minutes without letting it color.
6. Pour in the warm milk a little at a time, whisking constantly, then add the thyme. Simmer gently, stirring, for 5 to 6 minutes, until thick enough to coat a spoon.
7. Whisk in the roasted garlic paste and the mustard until smooth.
8. Take the pan off the heat and stir in the cheddar a handful at a time until melted and glossy.
9. Season with the nutmeg, black pepper and salt. Spoon over sliced potatoes, cauliflower or other vegetables for a gratin, or keep warm over low heat, whisking in a splash of water if it thickens.