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 9x13-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.