Preheat oven to 190 degrees. Soften the cabbage leaves in a bowl of just boiled water for 10 mins while you prepare the filling. Cut the sausage skin, squeeze out the meat and mix in a bowl with the finely sliced onion. Put a portion of the mixture into each cabbage leaf and wrap around the filling. Pour the tomatoes into an ovenproof dish and season with salt and a dash of vinegar. Place the wraps into the dish and drizzle over the olive oil. Roast in the oven for 30 mins, checking that the filling is cooked through