Throw the butter in a microwave-safe measuring cup. Nuke it 30s - 1m, to melt it. Stir in the flour until it’s smooth. Slowly stir in the milk a little at a time, until you’ve got a thick liquid. Microwave on high for 1m. Stir the lumps out. Add salt and pepper (and anything else you want to add) and stir some more. Microwave for another 30s-1m and stir some more. Let cool for a couple of minutes, so it finishes thickening.