Heat one tablespoon of the olive oil and one tablespoon of butter in a large skillet or pan over medium-high heat. Once the butter has melted, add in the chicken breasts and let the chicken cook for a few minutes on each side until golden brown. Remove from the skillet and onto a plate.
Add one more tablespoon of olive oil and add in the onions. Sauté the onions for a few minutes, or until translucent. Add in the garlic and mix to combine for about 30 seconds to 1 minute, scrapping up any browned bits that may be in the pan. Add in the mushrooms and cook until tender and have softened, for about 5 to 8 minutes. Season with salt and pepper to taste.
Add in the sun dried tomatoes and the heavy cream and mix to incorporate. Add in the spinach and mix and let cook until all the spinach wilts. Your sauce should be simmering. Carefully place each chicken breast into the skillet and nestle down into the sauce so that the sauce is surrounding each chicken breast. Cook until chicken is fully cooked through, (flipping the chicken breasts halfway through) and no longer pink inside. Use a meat thermometer to check that the internal temperature is 165˚F (74˚C).
Top the chicken with freshly grated parmesan cheese. You can also do a blend of Parmesan and mozzarella cheese. Feel free to add more cheese if desired. Cover the skillet or pan with a lid if needed. This will also help the cheese to melt faster. For garnish (optional): I added a few fresh basil leaves and some cherry tomatoes. Serve the chicken with a side arugula salad, mashed potatoes or with pasta. Serve and enjoy!