Preheat your oven to 375°F (190°C).
In a large skillet, cook the ground beef or turkey over medium heat until browned. Add the diced onion and minced garlic, and cook until the onion is translucent.
Stir in the diced tomatoes, black beans, corn, chili powder, cumin, salt, and pepper. Cook for another 5-7 minutes, until heated through and well combined.
Transfer the meat mixture to a baking dish and spread it out evenly.
Spread the mashed potatoes over the meat mixture, covering it completely.
Sprinkle the shredded cheddar cheese over the mashed potatoes.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Garnish with chopped cilantro before serving, if desired.