Alice's Piled On Roasted Potato Casserole

ALICE’S PILED-ON ROASTED POTATO CASSEROLE2 large Vidalia onions, thinly sliced6 large Portabella mushroom caps, stem and gills removed1 lb. bag of small variety of potatoes, washed1 (16-oz.) fresh pico de galloSalt, black pepper, garlic powderExtra virgin olive oil½ c. fresh…