I use white sauce for the Broccoli soup, scalloped potatoes, potato soup, chicken a la king. It comes in handy for making hamburg or sausage gravy too.
First, melt 1/2 stick margarine in sauce pan. Add 1/2 cup flour. With a fork blend until all flour in gone. This is the secret part. Slowly add 2 cups milk while stirring vigorously with a whisk. Stop adding milk periodically until mixture thickens then add more. It’s sort of like trying to get the lumps out of a can of tomato soup. Start with a little milk. It’s easier to season the sauce with salt and pepper before adding it to the other items.