Preheat your oven to 400°F (200°C).
In a large bowl, combine sliced kielbasa, onion, bell peppers, and pierogies.
Drizzle the mixture with olive oil and sprinkle with garlic powder, salt, and pepper. Toss to coat everything evenly.
Spread the mixture evenly on a large sheet pan.
Roast in the preheated oven for 25 to 30 minutes, stirring halfway through.
Once the pierogies are golden brown and the kielbasa is sizzling, remove from the oven.
Garnish with fresh parsley and serve warm.