Wash broccoli and place it in a large skillet. Add water and bring to a simmer over medium heat.
Cover broccoli and steam 3-5 minutes until tender-crisp. Drain & place in a bowl. Cover to keep warm.
Meanwhile, in a large microwave-safe bowl heat the butter for 30 seconds or until melted. Stir in the flour.
Gradually whisk in the warmed milk. Microwave on HIGH for 3-4 minutes, stopping after each minute to stir, until thickened. (Keep an eye on it so it doesn't bubble over).
Add cheese and microwave for 1 minute more, until cheese is melted. Stir until the cheese is fully combined and the sauce is smooth. Serve over broccoli.
Notes
Warm milk just slightly in the microwave (to take away the chill of the fridge) so that it doesn't harden the melted butter when combined and create a lumpy sauce.Leftovers will keep for 4 days in an airtight container in the refrigerator. It is not recommended to freeze.