Spray 3- to 4-quart slow cooker with cooking spray. In slow cooker, stir together broth, lentils, prechopped vegetables, oregano and pepper; add turkey leg.
Cover; cook on Low heat setting 8 to 10 hours or until lentils are tender and turkey falls off bone.
Remove turkey leg from slow cooker. Remove and discard skin. Shred meat; discard bone. Return meat to slow cooker; stir well. Ladle soup into 8 bowls; garnish with yogurt and oregano sprigs.