Melt the butter in a heavy-based saucepan over a low heat. Remove from the heat and stir in the flour. Gradually pour in the milk, stirring or whisking constantly.
2.
Return the pan to the heat and bring to the boil, still stirring or whisking. Reduce the heat and simmer the sauce gently for 5 minutes, stirring occasionally. Taste and season with salt and pepper. Serve or use immediately.