Preheat oven to 350 degrees F (175 degrees C).
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes; drain.
Mash potatoes with 2 tablespoons margarine; stir in milk. Season with salt and pepper.
Heat 2 tablespoons margarine in a large skillet over medium heat; stir in onions and saute until onions are translucent, about 5 minutes. Spread onions evenly over bottom of a 9×13-inch baking dish.
Cook and stir ground beef in same skillet over medium heat until no longer pink, breaking up large pieces, about 5 minutes. Layer beef into baking dish evenly over onion layer.
Wipe out skillet; place corn and creamed corn in skillet over medium heat and cook and stir until bubbly, about 5 minutes. Pour corn mixture over beef in baking dish. Spread mashed potatoes over corn mixture; dot with remaining 4 tablespoons margarine. Sprinkle with salt and pepper.
Bake in the preheated oven until casserole is bubbly and lightly browned at edges and margarine is melted, about 30 minutes.
You can substitute butter for margarine if you prefer.
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