I've picked up three main tips when it comes to fixing a lumpy bechamel sauce. Add egg: I know this might sound add. However, if you find that the sauce is a bit lumpy, then allow it to cool slightly before mixing in a beaten egg yolk (this is optional!). Alternatively, you can use an immersion blender to break up any lumps. This is a tip I picked up from a friend who often makes Greek-style Besamel (where eggs are a common ingredient). Add ice water: As you add in the milk, if you find it becoming too lumpy, add in a splash of ice water instead of milk and whisk – FRANTICALLY. The lumps will magically disappear. Use an immersion blender: The easiest of all three options. Simply stick an immersion blender into your sauce and whizz away the lumps. No one will ever know – shhh, I'll keep your secret!