Cauliflower Gratin

Preheat the oven to 400°F (205°C) and set an oven rack in the middle position.

Bring a large pot of well-salted water to a boil—it should taste like the sea. Add the cauliflower and cook for 5 minutes; it should be very al dente at this point, since it will continue cooking in the oven. Drain well and set aside.

Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1 to 2 minutes, until bubbling but not browned. Gradually whisk in the warm milk. Bring the mixture to a gentle boil, whisking often and occasionally running a flat whisk or rubber spatula around the edges of the pan to keep the sauce smooth. Reduce the heat and simmer, whisking frequently, until thick enough to coat the back of a spoon, 3 to 5 minutes. Add 1 teaspoon salt, ¼ teaspoon pepper, the nutmeg, ½ cup (60 g) of the Gruyère, and 2 tablespoons of the Pecorino Romano (or Parmigiano Reggiano); whisk until smooth.

Spread about one-third of the sauce over the bottom of a 2-qt or 8-in (2-L or 20-cm) square baking dish. Arrange the par-cooked cauliflower evenly on top, then spoon the remaining sauce over the florets. Sprinkle with the remaining ½ cup Gruyère.

In a small bowl, toss the panko with the olive oil, the remaining 2 tablespoons Pecorino Romano (or Parmigiano Reggiano), the thyme, and a pinch of salt and pepper. Sprinkle evenly over the top of the cauliflower.

Bake, uncovered, for 25 to 30 minutes, until the topping is golden brown and the sauce is bubbling around the edges. Let cool for at least 10 minutes before serving to allow it to set up.