Creamy Watercress Soup

Name: Creamy Watercress Soup
Ingredients: 2 tablespoons crumbled blue cheese (optional); 4 cloves garlic, minced; 1/2 cup half-and-half; 1/8 teaspoon salt; 2 tablespoons extra-virgin olive oil, divided; 4 cups reduced-sodium beef broth or vegetable broth, divided; 2 tablespoons shredded fresh horseradish (see Note) or prepared horseradish, or to taste; 8 cups chopped watercress, any tough stems removed, plus 1/2 cup leaves for garnish; 2 slices day-old sourdough bread, crusts removed, finely chopped; 3 tablespoons all-purpose flour; Freshly ground pepper to taste; 1 small onion, diced