Roast the Sweet Potatoes:
Heat oven to 375 degrees. In a small bowl, combine chili powder, cumin, garlic powder, onion powder, and ½ tsp salt. Set aside. In a medium bowl toss diced sweet potatoes and onions with olive oil until all vegetable pieces are coated. Sprinkle spice mixture over sweet potatoes and toss well to coat. Place mixture on a sheet pan and spread out into a single layer. Roast potatoes and onions in the oven for 30 minutes or until sweet potatoes begin to brown, tossing half way through the baking time. Remove from the oven.
Make Cilantro Lime Cream:
Place cilantro, sour cream, lime juice and remaining ½ tsp salt in a food processor or high speed blender. Blend until mixture is smooth. Pour into a small dish and refrigerate until ready to use.
Heat oven to 350. Spread chips out on a sheet pan or rimmed baking sheet. Sprinkle chips with 1 cup of shredded cheese. Top with black beans, corn, and roasted sweet potatoes. Cover with remaining 1 cup of shredded cheese. Place nachos in the oven for 5-7 minutes or until the cheese is completely melted. Remove from the oven.
Sprinkle nachos with tomatoes, and avocado. Drizzle cilantro lime cream over top of the nachos. Garnish with cilantro, if desired. Serve immediately.