01 - Set the oven to 400°F (200°C) to preheat.
02 - Boil a large pot of salted water, add cauliflower florets and cook 5 to 6 minutes until just tender. Drain thoroughly and set aside.
03 - Melt butter in a medium saucepan over medium heat, stir in flour and cook for 1 to 2 minutes, stirring constantly to form a roux.
04 - Gradually whisk in the milk to the roux, stirring continuously until sauce thickens, about 3 to 4 minutes. Remove from heat and mix in Dijon mustard, grated cheddar, Parmesan, salt, pepper, and nutmeg if using. Stir until smooth.
05 - Place the cooked cauliflower florets evenly in a baking dish. Pour the cheese sauce over the cauliflower, then sprinkle the remaining cheddar cheese and breadcrumbs on top if desired.
06 - Bake in the preheated oven for 20 to 25 minutes until the top is golden and bubbling.
07 - Allow to cool for a few minutes before serving to let flavors settle.