• Heat the oven to 375°F (190°C) and grease an approximate 10 x 10-inch baking dish (a 9 x 13 will work but yield a thinner casserole).
• Break the cauliflower into florets and cook them briefly—either steam or simmer in boiling water for about 5 minutes—until just starting to become tender. Drain well and set aside.
• Make a roux by melting 4 tablespoons of butter in a saucepan, then whisking in the flour until the mixture is smooth and bubbling.
• Slowly pour in the milk while stirring constantly, and cook until the sauce thickens enough to coat the back of a spoon.
• Remove the pan from heat and stir in 1 cup of grated Gruyère, the salt, pepper, and nutmeg until the cheese melts and the sauce is smooth.
• Spoon about one-third of the cheese sauce into the prepared baking dish to create a bottom layer.
• Arrange the par-cooked cauliflower over the sauce in the dish and season lightly with a pinch of salt.
• Pour the remaining creamy Gruyère sauce evenly over the cauliflower so it’s well coated.

For Topping:

• Combine the melted butter, panko breadcrumbs and the additional 1/3 cup grated Gruyère in a small bowl until the crumbs are moistened and the mixture is uniform.
• Sprinkle the breadcrumb-cheese mixture evenly over the sauce-covered cauliflower.
• Bake the casserole for about 30 minutes, or until the topping is golden and crisp and the cauliflower is tender when pierced. Let rest a few minutes before serving.