Preheat oven to 400F. Add 1 tablespoon of butter and 1 tablespoon of olive oil to a saucepan over medium heat. Add in the mushrooms and sauté for a few minutes until they have some good color.
Remove and add a little more butter to the pan, then sauté the garlic for about 20 seconds, careful not to burn it!
Add in the rest of the butter, chicken stock, heavy cream, mascarpone cheese, salt and pepper and stir until melted. Add in the parmesan cheese and parsley, continue stirring until everything has melted. In a large mixing bowl, add in the chicken, mushrooms and spinach.
Stir in the 1 cup of the shredded mozzarella cheese. Pour the sauce over the top and mix to combine. Pour the mixture into a 9" X 9" casserole or glass baking dish that has been sprayed with nonstick spray.
Spread the mixture evenly and top with the remaining mozzarella cheese and bake for 15 minutes. Increase the temperature to 425F and bake for another 10 minutes or until the cheese is golden.
Allow to cool slightly before serving. Due to the amount of healthy fats in this casserole, I recommend having small portions with a side salad to lighten things up a little. Store leftovers in the fridge for a few days or freeze.