Stir in the flour and cook for 1 minute, stirring constantly.
Gradually add the milk, whisking constantly, until you have a smooth sauce.
Whisk in the cheese for a minute or so until melted.
Notes
Whisk well when adding the milk, and be sure to add it gradually. This will ensure you have a smooth cheese sauce with no lumps.Store: You can store your cheese sauce in an airtight container in the fridge for up to 4 days.Reheat: Reheat it in the microwave or in a pot on the stove. You may need to whisk in a little milk when reheating if it gets too thick.Freeze: You can freeze leftover cheese sauce in an airtight container or ziplock bag for up to 6 months. Thaw then reheat, as above.