Preheat oven to 350 degrees.
In a large skillet melt 2 TBS butter over medium high heat. Add in chopped onion and soften. Slice sausage into 1/4 inch pieces and place into skillet with onion. Cook sausage until lightly browned. Turn off heat.
In a medium bowl whisk soup, milk, and seasonings (salt, pepper, garlic powder, onion powder, smoked paprika.)
Spread melted butter in a 9 x 13 baking dish evenly. Place frozen pierogi in a layer in bottom of baking dish overlapping if needed.
Place sausage & onion all over the pierogie. Pour soup mixture over the sausage, onion, and pierogie.
Cover baking dish tightly with foil. Bake in preheated oven for 30 minutes. Remove foil from baking dish. Sprinkle cheese all over casserole. Place baking dish back in oven uncovered for 15-20 minutes until cheese has melted and casserole is bubbly. Garnish with fresh parsley.