Melt the margarine or butter in a saucepan over a medium heat. Sprinkle in the flour and cook and stir for a few minutes until golden brown. Increase the heat and gradually add the milk, whisking constantly to prevent lumps, until thickened. Reduce the heat and allow to bubble gently for a few minutes. Stir in the grated cheese and black pepper, and continue stirring until the cheese has melted. Use immediately.
Add fresh herbs to the sauce, like chopped parsley or basil.