Heat the oil in a large, shallow saucepan and fry the sausages over a medium heat until golden and cooked through. Transfer them to a plate.
Meanwhile, thinly slice the onion and garlic, then the carrot. Peel and dice the sweet potatoes.
After removing the sausages from the pan, reduce the heat and add a drop more oil, if needed.
Saute the onion and garlic for 5 minutes until softened, stirring regularly.
Add the tomato purée and stir for a few seconds, then add the stock, tinned tomatoes, lentils and sweet potato & carrot pieces.
Bring to the boil then simmer, uncovered, for 20 minutes or until the veggies are tender.
Cut the sausages into bite-sized pieces and add them to the pan together with most of the parsley. Season with salt and pepper, if needed, and simmer for another 5 minutes to warm through.
If you want a thicker stew, mash a few of the sweet potato pieces with a fork.
Serve with the remaining parsley scattered on top.