Heat the oven to 425 degrees. Coat a 13x9-inch baking dish with non-stick cooking spray. Toss the sweet potato, onion, bell pepper and oil in a large bowl to coat. Arrange on a large rimmed baking sheet. Bake for 20 to 25 minutes or until the vegetables are tender and lightly browned. Reduce the oven temperature to 350 degrees. Combine the Pico de Gallo and 1/2 cup sour cream in a small bowl. Arrange 6 tortillas over the bottom of the baking dish. Layer with half each of the roasted vegetables, black beans, corn, enchilada sauce, sour cream mixture, cilantro and cheese. Repeat the layers. Bake for 20 to 25 minutes or until the cheese is melted and the casserole is hot. Sprinkle each portion with the green onions and serve with a dollop of the remaining sour cream.