Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.

In a skillet over medium heat, add the ground beef and chopped onion. Cook, breaking up the beef, until browned and the onion is translucent, about 6–8 minutes. Drain excess fat.

Return the skillet to low heat. Stir in the cream of mushroom soup and season with salt and pepper. Mix until combined and warm. Taste and adjust seasoning.

Spread the beef mixture evenly in the prepared baking dish.

Sprinkle the shredded cheddar evenly over the beef layer.

Arrange the frozen tater tots in a single even layer on top of the cheese.

Bake for 30–35 minutes, until the tots are golden and crispy and the casserole is bubbling around the edges. If you want extra browning, broil for 1–2 minutes at the end — watch closely.

Let the casserole rest for 5 minutes before serving so it sets up a bit.