Sieve flour into a bowl, season, and whisk in eggs and milk to form a smooth batter.
Rest the batter in the fridge for one hour.
Preheat the oven to 200°C.
Cook sliced onions in oil with salt for 20 minutes until dark golden-brown.
Stir mustard, pepper, and stock into the onions and simmer until reduced by half.
Heat a roasting tray in the oven, then brown the sausages in olive oil.
Pour the batter over the sausages and bake for 35 minutes until risen and golden.
Wilt shredded cabbage in water for 8 minutes, then drain.
Fry the cabbage with olive oil and garlic for 3 minutes until aromatic.
Reheat gravy and serve in wedges with the garlic cabbage.