Heat oil in large pot with lid. Add broccoli, thyme, onion, apple and celery. Cover and cook over low heat 10 minutes. Add chicken stock and pepper. Cook 30 minutes more. Remove from heat and cool slightly. Puree 1 cup at a time in blender or food processor. Serve hot or chilled, garnished with a dollop of yogurt and sprinkling of minced parsley. Makes 5 1/2 cups.