Step 1: Melt butter in a saucepan.
Step 2: Add flour, and stir in with a wooden spoon until it forms a paste, for about a minute.
Step 3: Take off the heat and add some milk, continuing to stir.
Step 4: Continue adding milk, while stirring all the time, until sauce has thickened.
Step 5: Add a dash of nutmeg if using.
Step 6: Put back on the heat to warm through.
Step 8: This can used to make many recipes, lasagna, fish pie, macaroni cheese, try adding different herbs and spices and add to chicken or fish.
Step 10: Experiment and enjoy!