Begin by breaking a large head of cauliflower into sizable florets, then boil or steam them for 5–7 minutes until just tender.

In a medium saucepan, melt ¼ cup of butter and sauté the onion until softened, then add flour to form a roux.

Gradually whisk in 2 cups of milk into the roux until boiling and thickened.

Stir in dry mustard, nutmeg, salt, and pepper, then remove from heat and add the cheese blend until melted.

Pour the cheese sauce over cauliflower in a baking dish, then mix remaining butter, panko, remaining cheese, and parsley for a topping.

Bake in a preheated oven at 400°F (200°C) for 25–30 minutes until golden brown and bubbling.