Preheat oven to 400 degrees. Coat 2 qt. baking dish with cooking spray. Coat med. pot with cooking spray, heat over med. heat. Add onion, cook until softened, about 5 mins. Add beans, mash with fork. Stir in chili and mexicorn, bring to simmer. Remove from heat, stir in 1/2 cup cheese. Pour mix into baking dish, top with tater tots in single layer. Bake 30 mins. or until bubbly. Sprinkle with remaining cheese. Bake until melted, about 2 mins. Let stand 10 mins. before serving.