Preheat the oven to 400°F. Lightly grease a 9x13-inch baking dish or coat it with nonstick spray.
Season the chicken thighs with Italian seasoning, salt, and pepper to taste.
In a large cast iron skillet over medium-high heat, melt 2 tablespoons of butter. Place the chicken skin-side down and sear both sides until golden brown, about 2-3 minutes per side. Remove and set aside.
Melt the remaining tablespoon of butter in the same skillet. Add the spinach and cook, stirring occasionally, until it begins to wilt, about 2 minutes. Set aside.
Arrange the seared chicken in a single layer in the prepared baking dish. Top with the halved potatoes, wilted spinach, and prepared garlic Parmesan cream sauce.
Transfer to the oven and roast until the chicken is fully cooked and reaches an internal temperature of 165°F, about 25-30 minutes.
Serve immediately, garnished with chopped parsley if desired.
For the Garlic Parmesan Cream Sauce
In the skillet over medium heat, melt the butter. Add the minced garlic and cook, stirring frequently, until fragrant, about 1-2 minutes.
Whisk in the flour and cook until lightly browned, about 1 minute. Gradually whisk in the chicken broth, thyme, and basil, continuing to whisk until smooth and incorporated, about 1-2 minutes.
Stir in the half-and-half and Parmesan, cooking until slightly thickened, about 1-2 minutes. If the sauce becomes too thick, add additional half-and-half as needed. Season with salt and pepper to taste.