Cornflour ....if you have some ..mix it with a little water and pour into the sauce a litle at a time until you get the required thickness..if no cornflour use ordinary flour and water but mix it well before you put it in the sauce or it will go lumpy. Bring the sauce up to the boil to cook the flour then turn it down to simmer .keep stirring it .
Got any cornflour? If so add 1 tablespoon to about 1 fluid oz of water,mix and empty into bol pan stirring. You can do the same with ordinary flour but you'll need to work hard at getting all the lumps out!