Prepare the Broccoli:
Place the broccoli in a large saucepan and cover with enough water to create a thin layer across the bottom of the pan (about 1 cup). Bring to a simmer over high heat, then cover the pan. Let the broccoli simmer until it’s bright green and crisp-tender, about 4-5 minutes. Drain the broccoli and place it in a bowl, covering it to keep warm.

Make the Cheese Sauce:
Wipe the pan dry with a towel, then add the butter and melt it over medium-low heat. Once melted, sprinkle the flour over the top and whisk constantly until the flour turns golden, about 1 minute.

Add the Milk:
Slowly pour in the milk while whisking constantly to avoid lumps. Let the sauce simmer for about 3 minutes, whisking continuously, until it thickens slightly.

Add the Cheeses:
Reduce the heat to low. Gradually add the shredded cheddar cheese in small handfuls, allowing it to melt fully between each addition. Stir in the Parmesan cheese, salt, mustard, and cayenne (if using). Taste and adjust the seasoning, adding more salt if desired.

Combine and Serve:
Serve the broccoli warm, topped with the cheese sauce, or gently stir the broccoli into the sauce and serve it mixed.