Brown the beef and onion in a skillet for 9 minutes until the beef reaches 160°F, then drain and transfer to a buttered 6-quart crockpot.
Brown the mushrooms in the butter for 6 minutes, stir in the flour for 1 minute, and gradually whisk in the milk, salt and pepper until thick.
Stir the mushroom sauce, corn and 1 cup cheddar into the beef.
Arrange the frozen tots in one layer over the filling, place a clean kitchen towel under the lid without letting it touch the food, and cook on high for 3 1/2 hours.
Scatter the remaining cheddar over the tots and cook uncovered on high for 20 to 30 minutes until melted and the potato edges are browned.