Mustard sauce
3 tblsp Butter
1 Onion, finely chopped
3 tsp English mustard powder
1 oz Plain flour
8 fl oz Milk
Salt and freshly ground black pepper
Melt the butter in a saucepan. Add the onion and cook over a very low heat for about 20 minutes, stirring occasionally. Whisk in the flour and mustard powder and cook over low heat for about a minute. Whisk in the milk a little at a time. Cook and whisk until the mixture bubbles and thickens. Allow to simmer for about 5 minutes. Taste and adjust seasoning with salt and black pepper.