Steam or microwave broccoli until just tender or cooked to your liking. Cover to keep warm.

Over medium heat, melt butter in a small saucepan. Add the flour and whisk until smooth. Cooks for a couple minutes, then whisk in mustard, pepper and onion powder.

Gradually add milk, whisking to keep sauce is smooth. Bring to a boil and cook until thickened, another minute or so.

Gradually add grated cheese, whisking the whole time. Remove from heat if getting too hot. You want it to melt but not boil. If the sauce gets too thick, or you just want a thinner sauce, feel free to stir in a little extra milk, a tablespoon at a time.

Taste and adjust salt. Once. Or twice.