Crush and finely chop the garlic! (or use a garlic press!)
Melt the butter in a large saucepan! Once melted, add the garlic and mustard powder and cook for 1 min! Stir in the plain flour, and then cook for another minute!
Gradually whisk in the milk until the sauce is lump-free! Simmer the sauce for 5 minutes, making sure that you whisk it the whole time!
Once thickened, take off the heat and stir in all of the primary cheese and the secondary cheese, and then season to taste! Use it in whatever you want! Enjoy! And as a side note, if you're looking to make a Béchamel sauce, simply don't add the cheese!
