Cook the whole sausages in a little more olive oil in a large pan until browned all over. Take the sausages out of the pan then add the onions and garlic and cook until softened.
Add the chilli (if using) carrot and celery, then cook for 5 minutes. add 2/3 of the rosemary, all of the tomatoes, wine or stock and lentils. Slice each sausage diagonally into 4 thick chunks then add back to the pan. Simmer with a lid almost covering for about 30 minutes, or until the lentils are tender.
The stew is best served with creamy potato puree but baked potatoes will be delicious too!