How to Make It
Preheat oven to 450°F. Heat oil in a 12-inch cast-iron skillet over medium-high. Add sirloin, and cook, stirring often to break up meat with a wooden spoon, until meat is no longer pink, about 8 minutes. Add onions and garlic, and cook, stirring often, until softened and aromatic, about 3 minutes. Drain beef mixture. Stir in corn, black beans, salt, and cumin; cook, stirring often, 2 minutes. Stir in pico de gallo, ketchup, and chopped chipotle chiles. Top mixture with grated cheese, and remove from heat.
Stir together mashed potatoes and salsa verde in a medium microwavable bowl. Microwave on HIGH until hot, about 3 minutes. Spread potato mixture in an even layer over cheese in skillet, covering entire surface. Spray lightly with cooking spray, and place skillet on a rimmed baking sheet.
Bake in preheated oven until potatoes are lightly browned on top, about 25 minutes. Remove from oven, sprinkle with cilantro and scallions, and serve hot.