Cook the ground beef and onion in a large skillet over medium-high heat, breaking the meat into small crumbles, until the beef is no longer pink and the onion is translucent, 6–8 minutes. Drain excess fat. Season with the kosher salt and black pepper.
Off the heat, stir in the condensed cream of mushroom soup, milk, and frozen mixed vegetables until evenly combined.
Spread the mixture into a 9Ă—13-inch baking dish in an even layer. Arrange the frozen tater tots in a tight single layer on top.
Bake until the filling is bubbling at the edges and the tater tots are deep golden and crisp, 45–55 minutes. Let rest 5 minutes before serving.