Bring 6 cups of water to a boil in a medium pot, and add chicken breasts. Cook 15 to 20 minutes or until a meat thermometer registers 165°F. Remove from water, and set aside to cool. Shred chicken, and place in a large bowl.
Preheat oven to 400°F. Spiralize sweet potatoes with the smallest noodle blade, and place them in a 13- x 9-inch baking dish.
Place tomatoes in a blender; cover and process until smooth.
Meanwhile, heat oil in a medium skillet over medium-high heat, and sauté onion, jalapeno, and garlic in hot oil until browned. Transfer to blender with tomatoes, and add cumin, chili powder, salt, and pepper. Cover and process until well blended.
Pour the tomato mixture into bowl with shredded chicken, and add corn and black beans. Mix to combine. Spread over sweet potatoes in baking dish, making sure the sauce mixes well and gets in between the spirals.
Cover the dish with aluminum foil, and bake 30 minutes.
Remove from oven, and top with cheese.
Bake, uncovered, 10 minutes longer or until the cheese is browned. Top with green onions, if desired.