Toss the cauliflower with olive oil, salt, and black pepper. Place them in a baking dish.
Roast the cauliflower in the preheated oven for about 20 minutes or until they are slightly golden and tender. Remove from the oven and set aside.
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk continuously to form a smooth paste (roux). Cook the roux for 1-2 minutes, but do not let it brown.
Gradually whisk in the warm milk, ensuring no lumps form. Continue to whisk until the sauce thickens. Season the bechamel sauce with salt, white pepper, and ground nutmeg. Remove from heat.
Pour the bechamel sauce over the roasted cauliflower in the baking dish. Sprinkle shredded Gruyere (or Swiss) cheese and grated Parmesan cheese on top. Sprinkle breadcrumbs.
Bake the cauliflower gratin in the oven for 15-20 minutes or until the top is golden brown and the sauce is bubbly.
Remove from the oven and let it rest for a few minutes. Serve warm.
