This recipe has three steps: baking a head of cauliflower until just tender, making the white cheddar sauce and final roasting to brown it all beautifully. You can cook the cauliflower and make the cheese sauce 2 days in advance and then reheat them together in a hot oven. Serve as a vegetarian main dish or a side dish for roast chicken, salmon or beef.
Preheat the oven to 400 degrees F. Use a paring knife to trim the leaves from the cauliflower. Cut around the main core to remove it while keeping the cauliflower in tact. Wash the cauliflower and shake it dry.
Place the cauliflower in a shallow baking dish. Rub it all over lightly with oil and season with 1/4 teaspoon of the salt. Cover with foil, pinching the foil around the edges of the baking dish to seal it tightly. Bake for 45 minutes.
Meanwhile, make the white cheddar sauce. Melt the butter in a sauce pan over medium heat. When it foams, add the flour and cook, stirring, for 2 minutes.
Whisk in the milk and bring to a simmer. Season with the remaining 1/2 teaspoon salt, black pepper, dry mustard and nutmeg. Whisk until smooth and thick enough to coat the back of a spoon, about 8 minutes.
Turn off the heat but leave the pan on the burner. Add the cheeses and stir until melted. Taste for seasoning.
Check the cauliflower, being careful about the steam when removing the foil, and test it with a table knife. If the core is still tough, leave the cauliflower in the oven uncovered for 10-15 more minutes until the knife slides through to the center easily.
Increase the oven temperature to 425 degrees F. Pour the white cheddar sauce over the cauliflower and bake uncovered until nicely browned, 20-25 minutes more. To serve, use a serving spoon to scoop portions, making sure to sweep through the additional cream sauce in the baking dish.