Preheat the Oven: Preheat your oven to 200°C (180°C fan)/Gas Mark 6.
Prepare the Cauliflower: Bring a large pot of salted water to the boil. Add the cauliflower florets and cook for about 5 minutes, until just tender. Drain well and set aside.
Make the Cheese Sauce: In a medium saucepan, melt the butter over medium heat. Stir in the flour and cook for 1-2 minutes to form a roux. Gradually add the milk, whisking constantly until the sauce thickens and becomes smooth.
Add Cheese and Seasoning: Remove the sauce from heat. Stir in the grated cheddar cheese, Dijon mustard, nutmeg (if using), and season with salt and black pepper to taste. Assemble the Dish: Place the drained cauliflower florets in a baking dish. Pour the cheese sauce evenly over the cauliflower. Sprinkle the grated Parmesan cheese on top if using. Bake: Bake in the preheated oven for 30-35 minutes, until the top is golden brown and bubbly
Garnish and Serve: Remove from the oven and let cool slightly. Garnish with chopped fresh parsley before serving.
