1. Heat olive oil in a large saucepan and gently fry the onion, garlic, carrots and celery for about 5 minutes, until soft.
2. Add lentils, chopped tomatoes, tomato paste, stock and marjoram and season to taste. Bring mixture to a boil and simmer for 20 minutes, partially covered, until the sauce thickens.
3. Meanwhile, add pasta to a large saucepan of plenty of salted, boiling water and boil, uncovered, for 8 – 9 minutes, until al-dente.
4. Drain the cooked spaghetti and empty into a warm bowl. Pour over the sauce, toss well and serve.