Ingredients

• 1 medium head (about 2 to 2 1/4 pounds) cauliflower
• 4 tablespoons (55 grams or 2 ounces) unsalted butter
• 4 tablespoons (30 grams or 1 ounce) all-purpose flour
• 2 teaspoons mustard powder
• Salt, to taste
• Freshly ground black pepper or ground cayenne
• 2 cups (475 ml) milk, whole is best but low-fat will probably work just fine
• 1 1/4 cups plus 2 tablespoons grated cheddar
• Chopped chives for garnish (optional)

1. Heat oven to 400 degrees.
2. Trim cauliflower and remove tough core. Cut into 1 to 2-inch florets. Steam (for about 10 minutes) or par-boil (6 to 7 minutes) florets until firm but tender. Drain, if needed, and spread florets on a towel so that it can wick out as much moisture as possible
3. Meanwhile, in a medium saucepan, melt butter over medium-high heat. Add flour and whisk to combine; cook for 1 minute to ensure you get rid of the floury taste. Add mustard powder and a pinch of cayenne or few grinds of black pepper, and stir to combine. Drizzle in milk in a thin, steady stream, whisking the whole time so that no lumps form. Season with salt and bring mixture to a simmer, stirring with a spoon; mixture should thicken. Stir in 1 1/4 cups cheddar, a handful at a time, letting each handful melt before adding the next. Taste sauce and adjust seasonings if needed.
4. Spread cauliflower florets in a 2-quart baking ovenproof baking dish. Spoon sauce over florets and sprinkle with remaining 2 tablespoons cheese. Bake until until bronzed and bubbly, about 35 minutes. 
5. Sprinkle with chives, if desired. Eat with abandon.