Step 1

Stir together the stock, water, sweet potato, carrots, russet potato, lentils, onions, celery, garlic, thyme sprigs, bay leaf, salt, and pepper in a 5- to 6-quart slow cooker. Cover and cook on LOW until the vegetables and lentils are tender, 7 to 8 hours. Remove and discard the thyme sprigs and bay leaf.

Step 2

Place the oil and 4 cups of the soup in a blender. Remove the center piece of the blender lid (to allow steam to escape); secure the blender lid on the blender. Place a clean towel over the opening in the lid (to avoid splatters). Process until smooth. Return the pureed soup to the slow cooker; stir in the vinegar. Ladle the soup into bowls, and sprinkle evenly with the parsley.