1. In a microwaveable bowl add the butter and melt for 30 seconds in the microwave.
2. Once done add the mustard, salt,pepper and plain flour. Whisk all the ingredients well to make a smooth paste.
3. Cook in the microwave for 30 seconds until flour is cooked.
4. Remove bowl from microwave. Gradually add milk while continuously whisking, to get a lump free smooth sauce. Return bowl to the microwave and cook for 1 minute. Remove and stir after every 20 seconds.
5. Repeat the process until the sauce thickens.
6. Add the grated cheese and stir well until it melts into the sauce.
7. You can use the cheese sauce to coat pasta or veggies or use it as a dip as well.