Heat the oven to 375°F (190°C) and butter a 2-quart gratin or baking dish. Bring a large pot of generously salted water to a boil.

Blanch the cauliflower florets for 5–6 minutes, just until they are tender enough to pierce but still hold their shape. Drain thoroughly and let them steam-dry for several minutes.

Melt 3 tablespoons of butter in a saucepan over medium heat. Whisk in the flour and cook for about 1 minute, stirring constantly.

Gradually whisk in the whole milk to prevent lumps. Simmer for 5–7 minutes, whisking frequently, until the sauce is smooth and thick enough to coat a spoon.

Season the béchamel with kosher salt, black pepper, and 1/4–1/2 teaspoon freshly grated nutmeg. Remove the pan from the heat and stir in 1/4 cup Gruyere and all of the Parmesan.

Arrange the dry cauliflower in the prepared baking dish and pour the cheese sauce evenly over the florets. Sprinkle the remaining 1/4 cup Gruyere over the top.

If using breadcrumbs, melt the remaining 1 tablespoon butter and toss it with the breadcrumbs or panko. Scatter the buttered crumbs evenly over the cheese.

Bake for 25–30 minutes, until the gratin is bubbling around the edges and golden on top. Broil for 1–2 minutes at the end if you prefer a darker crust, watching closely.

Let the gratin rest for 5–10 minutes before serving. This brief rest allows the sauce to set slightly and makes the dish easier to spoon.