Preheat the oven to Fan 200˚C/ Gas mark 7 / 425˚F.
To make the caramelised red onions, heat the oil in a large frying pan and add the onions and cook on a low heat for 30 minutes. Stir occasionally to prevent them from catching or burning until they become soft. Now add the sugar and vinegar and stir. Keep cooking on a low heat for another 5 minutes, stirring occasionally until the mix is sticky, the sugar has dissolved, and the vinegar has reduced. Leave to one side.
Put the sausages in a 20 x 30cm roasting tin with the oil and bake for 25 minutes until browned.
Meanwhile make the Yorkshire pudding batter. Tip the flour and salt into a large bowl, add the eggs and beat well. Slowly add the milk and water, constantly whisking until smooth. Leave to stand until the sausages are ready.
Remove the sausages from the oven. Be careful because the fat will be hot – but if it isn’t, put the tin back in the oven (without the sausages) to get the oil smoking hot. If you feel there isn’t enough oil in the tin (should be approve 3-4tbsp) add a little extra and get hot.
Pour in the batter, and place back into the oven and cook for 20-25 minutes, until risen and golden. Serve with the caramelised onions.