Preheat your oven to 350°F (175°C).
In a frying pan, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Add the sliced kielbasa to the frying pan and cook until browned, stirring occasionally, for about 5-7 minutes. Remove from the heat.
In a large mixing bowl, combine the sliced potatoes, heavy cream, garlic powder, paprika, salt, and black pepper. Mix well until the potatoes are evenly coated.
In the prepared baking dish, layer half of the potato mixture, followed by half of the kielbasa mixture. Sprinkle half of the cheddar and mozzarella cheeses over it.
Repeat the layers with the remaining potato mixture, kielbasa mixture, and top it off with the remaining cheeses.
Cover the baking dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 15 minutes until the cheese is bubbly and golden, and the potatoes are tender.
Once done, let it rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.