01 -
Place the broccoli in a large saucepan with enough water to create a thin layer across the bottom (about 1 cup). Bring to a simmer over high heat, then cover and cook until the broccoli is bright green and crisp-tender, 4 to 5 minutes. Drain, place in a bowl, and cover to keep warm.
02 -
Carefully wipe the pan dry with a towel. Melt butter over medium-low heat. Once melted, sprinkle flour over the top and whisk constantly until golden, about 1 full minute.
03 -
Slowly pour in the milk, whisking constantly to prevent lumps. Let simmer while whisking continuously until the sauce thickens slightly, about 3 minutes.
04 -
Reduce heat to low. Gradually add the cheddar cheese in small handfuls, allowing each addition to melt completely before adding more. Stir in the Parmesan, salt, and mustard and cayenne if using. Taste and adjust seasoning as needed.
05 -
Either pour the cheese sauce over the warm broccoli or gently stir the broccoli into the sauce before serving.