The secret to this onion gravy recipe is the long slow cooking of the onions and the best quality stock you can find.
Heat the oil in a frying pan and add the onion. Fry on a low heat for 30-45 minutes, stirring occasionally, or until the onion is golden-brown but not burnt.
Pour in the wine and stock, reduce the heat and allow to simmer for 20 minutes, or until the liquid has reduced by half. Stir in the mustard and season, to taste, with salt and freshly ground black pepper.