Step 1: Sear the Chicken to Perfection
Preheat your oven to 375°F (190°C). Pat the chicken thighs completely dry with paper towels. Season generously on both sides with salt, pepper, and Italian seasoning. In a large, oven-safe skillet or Dutch oven, heat the olive oil over medium-high heat. Add the chicken, skin-side down, and sear without moving for 5-7 minutes until the skin is golden brown and crispy. Flip and sear the other side for 2-3 minutes. Remove the chicken to a plate. It will not be cooked through.

Step 2: Build the Flavor Foundation
Reduce the heat to medium. In the same skillet with the chicken drippings, add the 3 tablespoons of reserved sun-dried tomato oil. Add the diced onion and sauté for 4-5 minutes until softened. Add the minced garlic and sliced sun-dried tomatoes, and cook for 1 minute until fragrant.

Step 3: Deglaze and Create the Sauce Base
Pour in the white wine to deglaze the pan, scraping up all the glorious browned bits (the fond) from the bottom. Let it simmer for 2-3 minutes until reduced by half. This step adds incredible depth. Stir in the chicken broth, heavy cream, dried oregano, and red pepper flakes. Bring to a gentle simmer.

Step 4: Incorporate the Parmesan and Spinach
Reduce the heat to low. Gradually whisk in the grated Parmesan cheese until it is fully melted and the sauce is smooth. Taste and adjust seasoning with salt and pepper. If using, stir in the fresh spinach until it just begins to wilt.

Step 5: Assemble and Bake
Nestle the seared chicken back into the skillet, submerging it partially in the sauce. If your skillet isn’t large enough, transfer the sauce and chicken to a greased 9×13 inch baking dish. If desired, sprinkle the optional mozzarella cheese over the chicken and sauce.

Step 6: Bake to Juicy, Bubbly Perfection
Transfer the skillet or baking dish to the preheated oven. Bake, uncovered, for 25-30 minutes (for thighs) or 20-25 minutes (for breasts), or until the chicken is cooked through (internal temperature of 165°F for breasts, 175°F for thighs) and the sauce is bubbly and slightly reduced around the edges. If using cheese, it should be melted and golden in spots.

Step 7: Rest, Garnish, and Serve
Remove the bake from the oven and let it rest for 5-10 minutes. This allows the juices to redistribute in the chicken and the sauce to thicken slightly. Garnish generously with fresh chopped basil and parsley. Serve directly from the skillet or dish, spooning plenty of the creamy sun-dried tomato sauce over each portion of chicken.