Ingredients
Melt the butter in a pan over a low heat. Add the onions with a pinch of salt and pinch of sugar to cook, stirring occasionally, until very soft and golden, 15-20 minutes.
If the onions start to crisp and dry out, add a splash of water. Stir in the flour then pour in the wine and stock, bring to the boil and let bubble gently for 15-20 minutes. Stir in the mustard and sauce and season to taste, adding a pinch or so of sugar if you like. Serve hot with bangers and mash.
Note; if you have no wine to hand, use 250ml stock or water and add 1 tbsp balsalmic vinegar or 1 tbsp red wine vinegar and ½ tsp sugar as well.