Steam the broccoli. If you have a preferred method, feel free to use that. I just put about an inch of water in a pot and add the broccoli in (water should not cover broccoli). Then cover the pot and put it on the stovetop over med-high heat for 5-8 minutes until the broccoli is bright green and tender (poke with a fork to check tenderness). I usually stir the broccoli once quickly while it’s steaming and then replace the lid. Drain the broccoli completely and transfer it to a 9x13 baking dish. Spread evenly throughout the dish and set aside. Pre-heat the oven to 350. In a medium saucepan, bring the butter over medium-low heat until melted. Add the flour a little at a time, whisking it into the butter until combined. Add the milk a little bit at a time, whisking it into the flour mixture until smooth and well combined. Turn the heat up to medium for a few minutes until the milk is warmed. Add the salt, pepper, mustard powder and 6 ounces of the shredded cheese (reserve 2 oz for topping) and whisk the mixture together. Continue to heat for another 3-5 minutes, whisking periodically, until the cheese is melted into the sauce and sauce has thickened. Pour the cheese sauce over top of the broccoli in the baking dish, making sure broccoli is evenly coated. Sprinkle the remaining cheese over the top. Place in the oven and bake for 20 minutes until cheese is melted and the dish is heated through. Allow a few minutes to cool before serving.