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, and drizzle over the broccoli.
Notes
Use any leftover sauce for more broccoli or for topping baked potatoes. Yum!