Place skinless bone-in thighs, leek, carrots, celery, garlic, broth, bay leaf, salt and pepper in a 6-quart slow cooker. Cover and cook on low for 5 to 6 hours until the chicken reaches 165°F and is tender.
Transfer thighs to a clean plate, discard the bay leaf and skim excess fat from the broth. Remove and discard all bones and any remaining skin or cartilage, then pull the meat into coarse shreds.
Turn the cooker to high, cover and heat the broth for 15 minutes. Stir in dry egg noodles and cover again.
Cook on high for 20 to 30 minutes until noodles are tender, stirring once after 15 minutes to separate them.
Return chicken to the soup and heat covered for 5 to 10 minutes until it reaches 165°F. Stir in dill and lemon juice and serve immediately in 6 bowls.