Cook the chicken: Heat olive oil or butter in a large skillet over medium heat. Season the chicken with salt and pepper, and sear it in the skillet for 5-7 minutes per side, or until cooked through and golden brown. Remove the chicken from the skillet and set it aside.

Sauté the onions and garlic: In the same skillet, add the chopped onion and garlic. Sauté for 3-4 minutes, until softened and fragrant.

Cook the spinach: Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes.

Make the creamy sauce: Stir in the chicken broth, heavy cream, Parmesan cheese, dried thyme, and parsley. Let the sauce simmer for 3-5 minutes, stirring frequently, until it thickens slightly.

Combine and serve: Return the cooked chicken to the skillet, spooning the creamy spinach sauce over the top. Let the chicken warm through in the sauce for 2-3 minutes. Garnish with fresh parsley and serve hot.