Rinse & drain split peas. Place in soup kettle & add broth, onion, celery, parsley, pepper & bay leaf. Cover kettle, bring to a boil, reduce heat & simmer for approximately 1 hour, or until peas are tender. Remove bay leaf. If smooth soup is desired, put through food mill or processor in small batches. Dissolve flour in water. With kettle on medium-high heat, add flour mixture to soup along with milk or half & half, stirring till thickened. Serves 6 to 8.