Preheat oven to 400°F (200°C) and grease a 2-quart baking dish.
Steam or boil cauliflower florets for 5-7 minutes until tender-crisp, then drain thoroughly and pat dry.
In a saucepan over medium heat, melt butter and whisk in flour for 1-2 minutes to create a light roux.
Gradually whisk in milk and simmer until the sauce thickens and coats the back of a spoon.
Remove from heat and stir in 1 cup of Gruyère, nutmeg, salt, and pepper until smooth.
Place the cooked cauliflower into the prepared baking dish and pour the cheese sauce evenly over the top.
Sprinkle with the remaining Gruyère, Parmesan, and breadcrumbs.
Bake for 20-25 minutes until the top is golden brown and the sauce is bubbling.