1. Place first 6 ingredients in a 3- to 4-quart electric slow cooker. Cover and cook on LOW 8 to 10 hours or until lentils are tender and turkey falls off the bone.
2. Remove turkey leg from cooker. Remove and discard skin. Shred meat; return to cooker, discarding bone. Ladle soup into bowls; garnish with yogurt and oregano sprigs, if desired.