Start by placing the chicken breasts between two pieces of plastic wrap. Using a meat mallet or rolling pin, gently pound the chicken to an even thickness, about 1 inch.
In a small bowl, mix together the garlic powder, kosher salt, ground black pepper, and onion powder. Season both sides of the chicken breasts generously with the spice mixture.
In a large skillet, heat the olive oil and butter over medium-high heat. Once the butter has melted and starts to froth, add the seasoned chicken breasts. Sear for about 5-7 minutes on each side, or until golden brown and cooked through. The internal temperature should reach 165°F (75°C).
Once the chicken is cooked, remove it from the skillet and set it aside on a plate. In the same skillet, pour in the low-sodium chicken broth, scraping up any brown bits from the bottom of the pan.
Reduce the heat to low and stir in the Boursin cheese until fully melted and combined with the broth. If using, add the fresh spinach at this point, allowing it to wilt into the sauce.
Return the chicken to the skillet, allowing it to soak up the creamy sauce for a couple of minutes. Serve hot, garnished with chopped fresh parsley.
