Heat the oven to 200C/fan 180C/gas 6. Cut the potatoes into 2cm chunks, toss in olive oil then spread out on a non-stick baking tray. roast for 40 minutes until crisp and golden, turning a couple of times during cooking.
Meanwhile, 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.
When the potatoes are cooked, toss with some sea salt and the rest of the rosemary, then serve with the stew.