Red-Leaf Salad with Roasted Sweet Potatoes

2 sweet potatoes, peeled and cut into 1-inch chunks
1 medium red onion, quartered
2 tablespoons olive oil
Coarse salt and ground pepper
12 oz. green beans, trimmed and cut into 2" lengths
1/3 cup walnuts, broken into large pieces
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

Preheat oven to 450 degrees. On a large baking sheet, toss together sweet potatoes, onion, and oil; season with salt and pepper. Roast until sweet potatoes are tender, about 20 minutes.

Add green beans to sheet; toss. Roast until green beans are tender, about 10-12 minutes. Add walnuts to sheet and toss again. Roast 5 minutes more.

Meanwhile, in a small bowl, whisk together yogurt, vinegar, and garlic; season dressing with salt and pepper. Top lettuce with roasted vegetable mixture; drizzle with dressing.