In a large saucepan, bring the broth, onion, and vanilla bean to boiling; reduce heat. Cover and simmer for 10 minutes or until onion is tender. Discard vanilla bean.
In a small bowl, stir together the butter and flour. Whisk into broth mixture. Cook and stir until slightly thickened and bubbly.
Add beans to broth mixture. Return to boiling; reduce heat. Cover and simmer for 5 to 7 minutes more or until beans are crisp-tender. Ladle into bowls. If desired, garnish with steamed green beans and serve with Herbed Sourdough Breadsticks. Makes 6 to 8 side-dish servings.
Herbed Sourdough Breadsticks
Preheat oven to 375 degree F. Cut half of a 2-inch sourdough baguette lengthwise into 1/2-inch slices. Cut into 1-inch-wide sticks. Brush cut surfaces evenly with olive oil and sprinkle with thyme. Place on baking sheet. Bake in the preheated oven for 16 minutes or until golden.
Steamed Green Beans
Place green beans in a steamer basket. Cover and steam over boiling water for 5 to 7 minutes or until tender.