Prepare the chicken by seasoning with salt and pepper on both sides.
Heat olive oil in a large skillet over medium-high heat. Add the chicken breasts and cook for about 5-7 minutes on each side until golden brown and no longer pink in the center.
Remove the chicken and set aside. In the same skillet, reduce heat to medium and add butter, followed by minced garlic and sliced mushrooms. Sauté until mushrooms are browned, about 5 minutes.
Pour in Marsala wine, scraping up any browned bits from the bottom of the skillet. Bring to a simmer and reduce for about 3-5 minutes until slightly thickened.
Add the spinach to the skillet and stir until it wilts, about 2-3 minutes.
Return the chicken to the skillet, turning it to coat in the sauce. Cook for another 2-3 minutes to heat through.
Optional: Sprinkle grated Parmesan cheese over the dish before serving for an extra layer of flavor.