Heat the oil in a large saucepan over medium-high heat. Cook the onion, stirring, for 2 minutes. Add the celery, carrot and fennel. Cook, stirring, for 3-4 minutes or until the vegetables are light golden.
Add the lamb and cook, stirring, for 2-3 minutes or until just browned. Add the tomato and stock. Bring to the boil. Reduce heat to low. Cover and simmer for 1 hour.
Add the lentils and potato. Cover and simmer for 1 hour or until the lamb and potato are tender. Ladle among serving bowls and top with a dollop of pesto.