Heat the oven to 375°F and grease a 9-by-13-inch baking dish.
Brown the beef and onion in a large skillet over medium-high heat for 8 to 10 minutes, breaking up the meat, until the beef reaches 160°F, then drain the fat.
Season the beef with the salt and pepper and spread it across the baking dish.
Whisk the soup and milk until smooth, fold in the green beans and corn, and spoon the mixture over the beef.
Arrange the frozen potato tots in one close, even layer over the filling.
Bake uncovered for 40 to 45 minutes, until the filling bubbles at the center and the tots are crisp and dark golden, then rest for 5 minutes.