How to Make It
Stir together broth, water, tomatoes, lentils, carrots, onion, celery, garlic, thyme, and marjoram in a 5-quart slow cooker. Cover and cook on LOW until lentils and vegetables are tender, 6 to 7 hours.
Transfer 2 cups lentil mixture to a blender. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening. Process until smooth, about 45 seconds. Stir back into mixture in slow cooker. Stir in vinegar, salt, and pepper. Ladle soup evenly into 6 bowls; sprinkle with parsley and cheese.