Put the onion, parsley stalks, peppercorns, grated nutmeg and milk in a large saucepan. Gently heat to simmering point, being careful not to let it boil, then remove from the hob and leave to infuse until the milk is cold.
Strain the milk into a clean saucepan, discarding the solids, then add the butter and flour. Gradually bring to simmering point, whisking continuously to create a smooth sauce. Continue to cook for 5-10 minutes until thickened. Your base white sauce is now ready to transform into a range of other dishes.