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Preheat oven to 375°F. Brown ground beef with onion in a large skillet, breaking meat into crumbles. Drain excess fat.
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Stir in cream of mushroom soup, sour cream, 1 cup cheese, and corn (if using). Season with salt and pepper.
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Spread mixture into a 9x13 baking dish. Arrange tater tots in a single layer on top.
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Bake 40-45 minutes until tater tots are golden and crispy. Sprinkle remaining 1 cup cheese on top and bake 5 more minutes until melted.
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Let rest 5 minutes before serving.