Preheat oven to 450 degrees.
Grease a 9X13 casserole dish.
In a large cast-iron skillet, melt 2 tablespoons of butter and saute the mushrooms, garlic, and onions on medium-high heat for approx 10 minutes until onions are translucent and mushrooms are very tender.
Remove from skillet and set aside.
Melt 3 tablespoons of butter in the same cast-iron skillet and sear chicken breasts or tenderloins for 3-4 minutes on each side so that they are nicely browned.
As each piece is browned, remove from the skillet and place it in the casserole dish leaving all of the drippings in the skillet.
Once all the chicken has been browned, add the mushroom mixture back to skillet along with chicken broth, heavy whipping cream, and salt/pepper.
Cook on medium-high heat, scraping the bottom of the skillet while stirring continuously for 5 minutes.
Turn off the stove. Sprinkle 1/8 tsp xanthan gum on top and allow the sauce to thicken. It will take 5-10 minutes for the sauce to thicken just a bit. It will still be a bit broth-y as you don't want it to thicken too much. You can also omit the xanthan gum as it does not change the taste of the dish. Once it has thickened spoon the sauce over the chicken in the casserole dish.
Sprinkle parmesan cheese over chicken first, then top with mozzarella.
Top with 1 tsp Italian seasoning.
Bake in a 450 oven for 12-14 minutes.
To brown mozzarella, move casserole dish to the top rack and turn oven to broil for 1-2 minutes.
Serve!