How to Make It
In a large, deep saucepan, bring 1 inch of water to a boil. Fill a steamer basket with the broccoli, add it to the saucepan, cover and steam over high heat until the broccoli is crisp-tender, about 5 minutes. Transfer the broccoli to a platter.
Meanwhile, in a saucepan, melt the butter. Add the flour and whisk over moderately high heat for 2 minutes. Add the milk and cream and cook, whisking, until thickened, about 2 minutes. Add the mustard and cheese and cook over moderately low heat until the cheese is melted. Season the sauce with salt, transfer to a bowl and serve with the broccoli.