Heat a large skillet to medium heat. Add in half the butter and olive oil, then add in the chicken. Sauté for a few minutes, until the chicken is seared on both sides, then remove and set aside. It will finish cooking in the oven.
Add in the mushrooms and sauté until they have some good color. Remove and set them aside, then add the garlic for about 20 seconds, careful not to burn it! Then add in the mascarpone and parmesan cheese, heavy cream, salt and stir.
Add the spinach, mushrooms and chicken to the sauce and stir until well combined. Preheat oven to 425F, while you simmer on low for about 5 minutes or until thickened to your liking. This sauce does not need to cook long at all, that's the beauty of it!
Top with the shredded mozzarella cheese and bake for 15-20 minutes or until the cheese is melted and slightly golden! Store leftovers in the fridge for a few days or freeze.
If you prefer to use fresh spinach, you certainly can. I would sauté it along with the mushrooms and follow the remaining steps of the recipe.