Preheat the slow cooker to Low then add the boiling water followed by the stock cubes. Stir to dissolve.
Add the lentils and vegetables and give a good stir. Season and cook on Low for 8 hours until the lentils and vegetables are tender. If desired, use a handheld liquidiser to puree the soup to make it smooth or enjoy it chunky. Serve in warmed bowls and enjoy with some crusty rolls.