Preheat oven to 375 degrees. Grease a 1½-quart baking dish with butter.

Bring a pot of salted water to a boil. Add the cauliflower florets and cook until crisp tender, about 5 minutes. Drain well.

In a large skillet over medium heat, melt 2 tablespoons of butter. Add the flour and cook, stirring, for 1 minute. Whisk in the milk, ¾ teaspoon salt, the pepper, and nutmeg. Cook, stirring frequently, until the sauce thickens, about 5 minutes. Add the Gruyere cheese and ½ cup Parmesan cheese and cook, stirring, until all the cheese melts.

Spoon a thin layer of the sauce into the prepared baking dish. Add the cauliflower in an even layer, and pour the rest of the sauce over top.

In a small skillet over medium heat, melt 2 tablespoons butter with 2 tablespoons olive oil. Add the panko bread crumbs and cook, stirring frequently, until the crumbs brown, about 5 minutes. Add the remaining ½ cup Parmesan cheese and sprinkle the mixture over the cauliflower.

Melt the remaining 2 tablespoons of butter and drizzle over the bread crumbs. Bake for 20 – 25 minutes or until the bread crumbs are golden brown and the sauce is bubbly.