Preheat oven to gas 7, 220°C, fan 200°C.
Put the flour in a bowl, add the eggs and slowly mix in the milk then beat until smooth.
Put the sausages in an ovenproof baking dish.
Scatter over one of the sliced onions.
Drizzle over the sausages and onion, about a tablespoon of oil.
Roast for 15 mins.
Remove the dish from the oven.
Pour the batter over and around the sausages then return to the oven.
Cook until the sausages are cooked through and the batter is golden on top. (Approx 30 mins)
Heat a saucepan and fry the remaining onion in some oil until golden.
Sprinkle over 2tsp of flour and cook, stirring until thickened.
Add the mustard, Worcestershire sauce and 300ml of vegetable stock.
Stir until the gravy is cooked to your preferred thickness.
Serve the toad in the hole with the gravy.