Preheat oven to 375 degrees. Toss diced sweet potatoes with oil, salt and cumin. Roast in a single layer on a baking sheet for 30 minutes. Combine roasted sweet potatoes, corn, black beans, green onion, tomatoes, and cilantro in a large bowl. Pour lime juice over everything. Stir, season to taste with salt a pepper. Serve. (Optional: add avocado just before serving). Leftovers will keep for up to 5 days in the refrigerator. Can be served hot or cold.