In a small saucepan, boil the sweet potatoes. Cook for approximately 15 minutes until tender. Remove from pan, peel off the skin, and cut into bite size cubes. Cook the green beans in the microwave for 2 minutes.
In a large bowl, combine the sweet potatoes with the green beans, onions, and turkey sausage. In a smaller bowl; combine the yogurt with cucumber and olives to make the salad dressing. Add the lettuce to the salad mixture just before serving. Pour some dressing, sprinkle the walnuts and serve.