Preheat oven to Gas 7, 220ºC, 425ºF. Place the flour in a bowl, add the eggs and slowly mix in the milk and beat until smooth.
Put the sausages in an oven-proof baking dish, scatter over the sliced onion and vegetable oil. Roast for 15 minutes. Remove from oven, pour over the batter and return to cook for a further 35 minutes.
To make the gravy, fry the remaining onion in the vegetable oil for 5 minutes until golden. Sprinkle on the flour and cook, stirring until thickened. Add mustard, Worcestershire sauce and, gradually, the stock, stirring until smooth. Serve with the toad in the hole and steamed veg.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.