Saute the onion, celery, carrots, bell peppers, and garlic in the oil until onion begins to brown(don't let the garlic brown). Add the zucchini and mushrooms and saute another couple minutes or so. Add the lentils, tomatoes, tomato paste, vegetable stock and spices. Bring to a boil, reduce heat to low and let simmer for 50 minutes. Taste to adjust seasonings. Add beans and cook until beans are heated through.