While the sausage röstis are baking, make the gravy. Put the onions and butter in a frying pan and cook gently until really soft and starting to caramelise – about 20-30 minutes. stir in the sugar, and brown over a higher heat for a couple of minutes, then stir in the flour, mixing until it has blended in. Gradually stir in the stock, followed by the remaining 1 tbsp grainy mustard and worcestershire sauce. Bring to a simmer and bubble, then reduce until it has a good consistency. Season to taste. Scoop the sausage röstis straight from the tin and serve with green veg and plenty of gravy.