- 2 tablespoons extra-virgin olive oil, divided
- 1 small onion, diced
- 4 cloves garlic, minced
- 4 cups reduced-sodium beef broth or vegetable broth, divided
- 3 tablespoons all-purpose flour
- 1/8 teaspoon salt
- 8 cups chopped watercress, any tough stems removed, plus 1/2 cup leaves for garnish
- 2 tablespoons shredded fresh horseradish (see Note) or prepared horseradish, or to taste
- 1/2 cup half-and-half
- Freshly ground pepper to taste
- 2 slices day-old sourdough bread, crusts removed, finely chopped
- 2 tablespoons crumbled blue cheese (optional)
Heat 1 tablespoon oil in a large Dutch oven over medium heat. Add onion and garlic and cook, stirring often, until the vegetables start to soften and brown, 3 to 5 minutes. Meanwhile, whisk 1 cup broth and flour in a small bowl until completely smooth. Set aside. Add the remaining 3 cups broth and salt to the pot and bring to a boil. Reduce heat to maintain a simmer and cook, stirring often, until the onion is very tender, about 5 minutes.
Stir chopped watercress into the pot and cook, stirring often, until tender, about 5 minutes. Stirring constantly, add the flour mixture and horseradish. Bring to a simmer and cook until thickened, 1 to 3 minutes.
Puree the soup in a blender in batches until smooth. (Use caution when pureeing hot liquids.) Return to the pot, stir in half-and-half and season with pepper; keep warm.
Heat the remaining 1 tablespoon oil in a medium skillet over medium-low heat. Add breadcrumbs and cook, stirring often, until golden and crispy, 3 to 5 minutes.
Ladle soup into 4 bowls. Garnish with the breadcrumbs, watercress leaves and blue cheese, if desired