01 - Set the oven temperature to 400°F (200°C).
02 - Bring a large pot of salted water to a boil. Add the cauliflower florets and simmer for 5 to 6 minutes until just tender. Drain thoroughly and set aside.
03 - Melt butter in a medium saucepan over medium heat. Stir in the flour to form a roux and cook for 1 to 2 minutes, stirring constantly.
04 - Gradually whisk in the milk to avoid lumps. Continue stirring over medium heat until the sauce thickens, about 3 to 4 minutes. Remove from heat and blend in Dijon mustard, grated cheddar, Parmesan, salt, pepper, and nutmeg if desired until smooth.
05 - Place cooked cauliflower evenly in a baking dish. Pour the cheese sauce over the florets, distributing it evenly.
06 - Sprinkle remaining cheddar cheese and fresh breadcrumbs, if using, over the sauce-covered cauliflower.
07 - Bake in the preheated oven for 20 to 25 minutes until the topping is golden and bubbling.
08 - Allow the dish to cool slightly for a few minutes before serving to enhance flavor and texture.