Heat the oven to 220C. Coil the sausage loosely and put in a large ovenproof dish with high sides. Pour over 1 tablespoon of oil and cook in the hot oven for 10 minutes. Put the flour into a medium bowl and make a well in the centre. Add the milk, eggs and salt and mix well to make a smooth batter.
Remove the dish from the oven and carefully pour the batter into the dish, and return to the oven for 25-30 minutes without opening the door.
Meanwhile make the gravy by gently sauteing the onions in the remaining 1 tablespoon oil for 20m until soft but not coloured, add 1 tablespoon brown sugar and 1 tablespoon balsamic vinegar, sweat until reduced and then add the gravy. Leave to bubble for 30 seconds and then pour over the cooked toad in the hole.