There are various ways to thicken a sauce. If you don't like roux and don't want to buy ready-made sauce thickener, you can use a simple trick to thicken a sauce without lumps.

 

Cook the sauce as usual. Mix a tablespoon of flour with a tablespoon of cold water in a cup. The flour dissolves quickly in cold water and becomes a smooth paste. Stir this flour paste into the simmering sauce and bring to the boil briefly. Then remove from the heat.