Preheat oven to 375F. Add the shredded chicken to a large bowl. Add in the mushrooms. Squeeze all the water out of the spinach, then add to the bowl with the chicken and mushrooms.
Heat a saucepan to medium and melt the butter. Add in the garlic and sauté for about 20 seconds, careful not to brown! Add in the heavy cream and stir.
Add in the mascarpone cheese, and stir until melted. Add in the grated parmesan cheese, salt and pepper. Turn off the heat and stir.
Pour the sauce over the bowl with the chicken, mushrooms and spinach. Mix, then add 1/2 cup of the mozzarella cheese and stir until well combined.
Pour into a casserole dish that has been sprayed with nonstick spray. Sprinkle the remaining 1 cup of mozzarella cheese over the top and bake for 25 minutes.
Turn the temperature up to 425F and bake for an additional 10 minutes or until the cheese is melted and golden. Allow to cool slightly before serving. Store leftovers in the fridge for a few days or freeze.
I used canned mushrooms in this recipe (something I rarely do), as a time-saver. Feel free to use fresh mushrooms, but be sure to sauté them first, then add them to the casserole. The same is true of the spinach. I used frozen spinach, but you can certainly use fresh, sauté first and add to the casserole.