Preheat oven to 375℉.
Grease baking dish.
Add frozen pierogi to the casserole dish in an even layer.
Top with diced ham. Set aside.
In a skillet, sauté diced onion with butter over medium heat until soft, about 5 minutes.
Spread the sautéed onions over the ham in the casserole dish. Set aside.
In the bowl of a stand mixer, combine the softened cream cheese, sour cream, milk, salt, pepper and garlic powder. Mix until well blended.
Add 1 cup of grated cheddar cheese and mix to combine all the ingredients together.
Spread the cheese mixture evenly over the Pierogi, ham and sautéed onions in the casserole dish.
Top with the remaining 2 cups of shredded cheddar cheese.
Cover dish with foil and bake for 30 minutes.
Remove foil, and bake for an additional 20-30 minutes or until heated through and cheese is hot and bubbly.
Remove from the oven and allow casserole to sit for 5-10 minutes before serving.
Top with sliced green onions to serve.