Melt the butter in a large skillet over medium-high heat, then add the ground beef and chopped onion, seasoning with salt and pepper while breaking the meat apart as it cooks.

Cook the mixture until the beef is fully browned and no pink remains, then continue stirring so the onions soften and blend into the meat.

Using a slotted spoon, transfer half of the cooked beef mixture into the bottom of a baking dish, allowing excess liquid to drain off before layering.

Add half of the vegetables over the beef, season lightly, then spread one can of cream of mushroom soup evenly across the top.

Repeat the layering process with the remaining beef, vegetables, and the second can of soup, spreading everything evenly for consistent texture.

Arrange the frozen tater tots in a single, even layer across the top, covering the entire surface so they bake up crispy.

Bake in the preheated oven until the tater tots are golden brown and the filling is bubbling around the edges.

Let the casserole cool slightly before serving so it holds together better when scooped.

Serve warm, optionally with ketchup on the side for added flavor.