In a medium sized skillet over medium heat, melt butter
add the onions and sautee until lightly browned and slightly translucent. Remove and cool slightly.
Spoon the sautéed onion evenly over the cheese; layer the chicken evenly over onions.
Have the tomatoes “paper toweled” off and slice thinly and sprinkle over the chicken and follow up with the julienned sliced Portabella mushrooms.
Sprinkle the Parmaggiano/Reggiano over the top and place pizza on pizza stone in a preheated 450 degree oven and bake for 10-12 minutes or until pizza is baked through and top is nicely browned and crispy.