Slice the fully cooked kielbasa into 1/2-inch coins and thinly slice the onion. Keep the pierogies frozen until the skillet is ready.

Heat the olive oil in a large skillet over medium-high heat. Add the kielbasa and cook for 4–6 minutes, turning occasionally, until browned on both sides. Transfer it to a plate.

Reduce the heat to medium. Add the butter and onion to the same skillet and cook for 6–8 minutes, stirring occasionally, until soft and golden.

Move the onions to the edges and add the frozen pierogies in a single layer. Add 1/4 cup water, cover, and cook for about 5 minutes, or until heated through according to the package directions. Uncover and cook for another 6–8 minutes, turning gently, until golden on both sides.

Return the kielbasa to the skillet. Add the garlic powder, black pepper, paprika, and red pepper flakes. Toss gently and cook for 2–3 minutes, until everything is steaming hot. Taste before adding any extra salt because kielbasa is already salty.

Divide among four plates and serve hot with sour cream and chopped parsley, chives, or green onions, if desired.