Place the washed lentils into a large saucepan or stockpot. Cover with the stock.
Bring to the boil and cook, uncovered for 10 minutes. Cover the pan, lower the heat and simmer for a further 15 minutes.
Place the lentils, stock, tomatoes (including any juice) and parsley into a food processor and blitz until smooth. Return the soup to the pan and reduce slightly or until a thickness you like.
Season with salt and pepper to taste.