Heat the oil in a large saucepan. Add the garlic, bacon, onion, carrots and swede and mix well. Cook over a medium heat, stirring frequently, for 5 minutes until the bacon has changed colour and the onion has softened.
Stir in the stock, lentils and half the lemon zest. Bring to the boil, then reduce the heat, half cover the pan and cook gently for 10 minutes.
Stir in the parsnips, cover and continue to cook gently for 25–30 minutes until the lentils and vegetables are tender.
Use a vegetable masher or hand-held blender to crush some of the lentils and vegetables into the soup, leaving the majority chunky. Stir in half of the parsley and season to taste. Serve sprinkled with the remaining chopped parsley and grated lemon zest.