Preheat oven to 450 degrees. On a large rimmed 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 and walnuts to sheet; toss. Roast until green beans are tender, about 5 minutes.
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 the yogurt dressing.