Heat oil in a large (8-quart or larger) soup pot over medium heat. Add onions, carrot, celery, and garlic. Sauté, stirring frequently, until softened, about 10-12 minutes. Add cabbage and cook, stirring occasionally, until slightly softened, about 10 minutes more. Add tomato paste and turmeric and cook, stirring, for 1 minute longer.
Add broth, water, cooked lentils, and salt. Cover and bring to a boil over medium-high heat. Reduce heat and simmer, partially covered, until vegetables are tender, about 10 minutes. Remove from heat and stir in marjoram and fresh lemon juice. Serve garnished with yogurt and avocado.