Peppery Potato Soup (Crock Pot)

2 cans chicken broth
4 small baking potatoes, halved and sliced crosswise
1 large onion, quartered and sliced
1 stalk celery, sliced
1/2 tsp salt
1/2 tsp black pepper
1 cup half and half
1/4 c all purpose flour
1 tbsp butter

Combine broth, potatoes, onion, celery, salt and pepper in Crock Pot and mix well. Cook on LOW 6-7 hours. Stir half and half into flour; stir mixture into slow cooker. Cover and cook one hour. Slightly mash potato mixture with potato masher. Cook uncovered 30 minutes or until slightly thickened. Just before serving, stir in butter. Garnish with celery leaves and parsley if desired.