Preheat the oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.
In a large skillet, cook the ground beef with the onion and garlic over medium-high heat until browned. Drain excess grease.
Stir in the taco seasoning with about ¼ cup water and simmer for 2 minutes.
Lower the heat and mix in the black beans, corn, cream of mushroom soup, sour cream, and 1 cup of cheddar cheese until creamy.
Spread the beef mixture evenly into the prepared baking dish and sprinkle the remaining cheddar cheese on top.
Arrange the frozen tater tots in a single layer over the cheese.
Bake uncovered for 45–50 minutes until the tater tots are golden and the casserole is bubbly. Rest a few minutes before serving.