Preheat the oven to 400ºF.

Place cauliflower core side down on a cutting board. Slice the cauliflower inot four 1/2-inch "steaks". Transfer the "steaks" and any loose florets to a baking sheet. Drizzle with the olive oil and the salt and pepper. Roast the cauliflower until it is tender and golden brown, about 20 to 25 minutes.

In a small bowl combine the Gruyere and Parmesan cheeses and set aside. In another small bowl, whisk together the cream, flour and nutmeg.

Transfer the cauliflower from the baking sheet to a 2 quart baking dish and arrange half of the cauliflower at the bottom of the dish. Sprinkle half of the cheese mixture over the cauliflower and then pour half of the cream mixture over the topol. Repeat with the remaining cauliflower, cheese and cream.

Bake the gratin until bubbly and golden brown, 35 to 40 minutes. Let the gratin stand for 5 minutes before serving.