1. Preheat oven to 400 degrees. Coat an 8-by-8-inch baking dish with cooking spray.
2. Combine pinto beans and their liquid, tomatoes, corn, chicken, 1/4 cup scallions, chili powder, cumin and salt in a large saucepan. Cook over medium-high heat, stirring often, until mixture is bubbling and corn is completely thawed, 3 to 5 minutes.
3. Layer about a third of the tortilla wedges, half the bean mixture and a third of the Monterey Jack in prepared baking dish; repeat. Top with remaining tortilla wedges and cheese. Bake until cheese is melted and casserole is heated through, about 12 minutes. Let cool slightly before serving; garnish with diced tomato and remaining scallions.