In a large saucepan, heat the oil and cook leeks and garlic 5 minutes.
Add carrots, bell pepper, mushrooms, tomatoes, stock, lentils, salt and pepper and mix well. Bring slowly to a boil, stirring, then cover and simmer 30 to 45 minutes, stirring occasionally, until lentils are cooked.
Add parsley and mix well. Garnish with parsley sprigs and serve with freshly cooked spaghetti or tagliatelle.