Position a rack in the lower third of the oven and preheat to 500°. In a large skillet, heat 1 tablespoon olive oil over medium heat. Add the kielbasa and cook, stirring, until lightly
browned, about 5 minutes; transfer to a medium bowl. Add the onions and remaining 1 tablespoon olive oil to the skillet and cook for 2 minutes. Lower the heat to medium-low, season with salt and pepper and cook, stirring occasionally, until golden-brown, 8 to 10 minutes.
Stir in the kielbasa and thyme and season with salt and pepper.