Whisk the batter ingredients together with a pinch of sea salt, and put to one side.
Put 2 tablespoons of sunflower oil into a baking tin, then place on the middle shelf of your oven at its highest setting (240–250ºC/475ºF/gas 9).
Place a larger tray underneath it to catch any oil that overflows from the tin while cooking.
When the oil is very hot, add the sausages – keep an eye on them and allow them to colour until lightly golden.
Take the tin out of the oven, being very careful, and pour the batter over the sausages.
Throw a couple of sprigs of rosemary into the batter.
Put the tin back in the oven, and close the door. Don't open it for at least 20 minutes.
Remove from the oven when golden and crisp.
Peel and finely slice the onions and garlic, then fry off in the butter on a medium heat for about 5 minutes, or until they go sweet and translucent.
Add the balsamic vinegar and allow it to cook down by half.
Add a stock cube or powder, sprinkle this in and add a little water.
Allow to simmer to create a tasty onion gravy.
