Cheesy Broccoli

Place broccoli in a saucepan with about 1 cup of water, bring to a simmer, cover, and cook until crisp-tender, 4-5 minutes. Drain and keep warm.
Wipe the pan dry, add butter, and melt over medium-low heat. Sprinkle flour and whisk for 1 minute until golden.
Slowly pour in milk, whisking constantly to avoid lumps. Simmer until thickened, about 3 minutes.
Reduce heat to low. Add cheddar cheese in small handfuls, whisking until fully melted. Stir in Parmesan, salt, mustard, and cayenne.
Taste and adjust seasoning. Serve broccoli warm with cheese sauce over the top or stirred in.