Prepare the Pan and Topping: Heat the oven to 425 F (220 C) and butter a 1 1/2-quart baking dish with 1 teaspoon of butter. Melt the remaining 2 teaspoons of butter and toss with the panko crumbs. Set aside.

Boil the Cauliflower: Bring a pot of water to a boil. Add the cauliflower. Boil for 5 minutes. Drain well and transfer it to the prepared baking dish.

Make the Sauce: Melt the 4 tablespoons of butter over medium heat in the same pot or a medium saucepan. When the butter sizzles, add the flour. Cook, whisking constantly, for 2 minutes. Gradually whisk in the milk and mustard (if using), and continue cooking while constantly whisking until thickened. Add 1 cup (about 4 ounces) of the shredded cheese and cook until thickened and the cheese has melted. Taste and season with salt and pepper.

Finish the Dish: Pour the cheese sauce over the cauliflower florets and top with the remaining cheese. Sprinkle with the buttered panko crumbs.

Bake: Bake the cauliflower cheese for 15 to 18 minutes or until bubbling and browned.

Serve: Serve as a side dish. Enjoy!