Wipe down your mushrooms with a damp paper towel. Remove the base of each mushroom and scoop out some of the center. Keep and finely chop your stems and the mushroom pieces you've scraped out.
Finely chop your Onion.
Saute mushrooms and onion with 2 Tbsp butter over medium/high heat stirring frequently for 10-12 minutes or until onions are soft and beginning to turn golden. Add 1/2 tsp salt.
Place cream cheese and well drained chicken in a medium bowl and season with 1/8 tsp freshly ground black pepper, then mash together with a fork.
Stir the mushroom-onion mix (it's ok if it's still very hot) into the cream cheese mix and mash together with the fork.
Use a rounded mini ice cream scoop or a teaspoon to fill the mushrooms, just enough for the filling to be domed over the top. P.S. If you have leftover stuffing, it makes for an incredible dip.
Finely grate your mozzarella cheese and place a pinch of it over the top of each stuffed mushroom.
Bake at 400˚ for about 10-12 minutes (mine took 12 min) or until they are soft to the squeeze and just starting to look a little juicy. Do not overcook them or they will get soggy. Serve hot. They also reheat really well in the microwave and taste just as amazing as they did straight out of the oven!