Heat olive oil in a large pan. Add onion, garlic, and celery; sauté until soft.
Add potatoes, carrots, and salt; cook for 1-2 minutes. Add lentils, thyme, marjoram, and vegetable stock. Bring to a boil, then simmer for 25-30 minutes until lentils are tender.
Remove from heat, stir in cheese, and dissolve sour cream in a small amount of soup before mixing back. Optional: blend part of the soup for creaminess. Season to taste and serve.
Notes
You can adjust salt based on your broth's saltiness. For a creamier texture, blend part of the soup.