Make the roux paste
Meanwhile, make the béchamel. Put ½ onion, 1 bay leaf and 500ml milk in a pan. Gently heat until just steaming, then set aside to infuse for 5 mins. Strain and discard the flavourings.
In another pan, melt 50g butter, then stir in 50g plain flour to make a roux paste. Cook over a low heat for 2-3 mins, stirring. Gradually whisk in the warm milk to make a smooth sauce. Simmer, stirring constantly, for 5 mins or until thickened.
Tip: Cooking the roux paste for the initial 2-3 mins before you slowly add the milk is essential as it makes sure that the flour becomes fully incorporated, which gives you a smooth, thick sauce.