Preheat oven to Gas 7, 220ºC, 425ºF. Place flour in a bowl, add eggs and slowly mix in milk to make a smooth batter. Add 150ml (¼pt) water and beat until smooth.
Put sausages in an oven-proof baking dish and scatter over the sliced onion. 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 remaining onion 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.