Red-Leaf Salad with Roasted Sweet Potatoes

INGREDIENTS

• 2 sweet potatoes, peeled and cut into 1-inch chunks
• 1 medium red onion, quartered
• 2 tablespoons olive oil
• Coarse salt and ground pepper
• 1 package (10 ounces) frozen cut green beans, thawed
• 1/3 cup walnuts
• 1 cup plain low-fat yogurt
• 2 tablespoons white-wine vinegar
• 1 garlic clove, crushed through a garlic press
• 1 head (10 ounces) red-leaf lettuce, torn into bite-size pieces