Sift flour and salt into a large bowl and make a well in the centre. Add the eggs and a little milk and beat until smooth, gradually beat in the remaining milk to make a smooth batter. Leave to stand for 30 mins. Preheat the oven to 220C/Fan 200C/Gas Mark 7. Divide the oil, sausages and onions between four shallow ovenproof dishes or 1 large roasting tin and toss to mix. Bake for 5 mins. Remove the sausages from the oven, turn them all over, then pour over the batter.
Bake for 25-30mins or until the batter has risen and is crisp and golden. To make the gravy: heat the oil in a medium pan, add the onions and cook for 5 mins or until golden brown. Add the sugar and cook over a low heat for 2 mins or until caramelised. Add the flour to the pan and cook, stirring for 1 min. Gradually add the stock, then bring to the boil and simmer for 1 min. Season to taste and keep warm.
Serve the toad in the hole with the onion gravy.