In a large skillet over medium heat, brown the ground beef until it is no longer pink. Drain any excess fat. Add chopped onion and sauté until translucent, then add frozen vegetables. Heat until softened. Season with salt and pepper.
Stir in the cream of mushroom soup until well combined. Remove from heat.
Preheat the oven to 375°F (190°C). Spread beef mixture evenly in a 9x13 inch baking dish, layer with half the cheese, and arrange tater tots over the top.
Cover the dish with foil and bake for 30 minutes. Remove foil and bake for an additional 15 minutes until golden and crispy. Sprinkle remaining cheese before serving.
Notes
For best results, store leftovers in an airtight container in the fridge. Reheat covered at 350°F until warm, about 15-20 minutes.