Preheat oven to 375°F and grease an 8x8 or 9x9 baking dish.
Sauté diced onion in a skillet over medium heat. Add ground beef and cook until browned. Drain excess grease.
Stir in jarlic, taco seasoning, Rotel, black beans, and corn. Simmer 5–7 minutes until thickened and fragrant.
Heat the mashed potatoes according to package directions. Stir in 1/2 cup of the shredded cheese and black pepper.
Layer: Add the taco meat mixture into the bottom of your baking dish. Top with the cheesy mashed potatoes and spread evenly.
Sprinkle the remaining 1 cup cheese over the top.
Bake uncovered for 20–25 minutes until bubbly and golden on top.
Garnish with chopped green onions or cilantro. Serve with sour cream and hot sauce.