You can either boil the cauliflower or roast it in the oven. Pick your favorite cooking method.Boiled: Cut the cauliflower into florets, then boil it in a large pot with lightly salted water until fork-tender (about 10 - 15 minutes). Drain it and set aside.
Roasted: preheat oven to 400°F or 200°C. Arrange florets on a baking sheet lined with parchment paper, season with olive oil and salt, and bake for 30 minutes, or until slightly charred and fork tender.
MAKE THE CHEESE SAUCE
In a saucepan, melt the butter, then add the flour and whisk for 1 minute or until it starts to bubble.
Add the milk and stir on medium heat until the sauce thickens.
Turn the heat off, then add salt, blackpepper, gratednutmeg, and shreddedcheese.
Stir until the cheese is fully melted in the sauce. Taste sauce and adjust.Tip: if you can't get the cheese to melt fully, you can blend the sauce with an immersion blender, or in a regular blender.
BAKE THE DISH
Preheat the oven to 350°F or 180°C. Transfer cauliflowerflorets onto a baking dish (11x8-Inch or 28x20cm).
Pour the cheese sauce over the cauliflower, and move the florets around so that the sauce goes into all nooks and crannies.
Sprinkle the grated parmesan on top.
Bake at 350°F or 180°C for 10 minutes, then set the oven to broiler function and gratin for 5 more minutes, or until the cheese on top melts and turns golden brown.