Preheat your oven to 375°F (190°C).

In a large skillet over medium heat, add the olive oil and butter. Once melted, add the sliced onions. Sprinkle with salt and pepper. Cook the onions, stirring occasionally, for about 20-25 minutes, or until they are golden brown and caramelized. If they begin to stick, add a splash of water to deglaze the pan.

While the onions are caramelizing, season the chicken breasts with garlic powder, dried thyme, salt, and pepper.

In a separate large skillet over medium-high heat, add a little olive oil. Add the seasoned chicken breasts and sear them for 3-4 minutes on each side until golden brown. They do not need to be cooked through, as they will finish cooking in the oven.

In a baking dish, arrange the seared chicken breasts. Top each piece with the caramelized onions, spreading them evenly. Sprinkle the grated Parmesan cheese over the onions and chicken.

Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through and the cheese is melted and bubbly.

Once done, remove the dish from the oven and sprinkle with fresh thyme leaves. Serve hot, ideally with a side of roasted vegetables or a fresh salad.