Phyllo Dough Sheets:
One 16-ounce package frozen phyllo dough sheets thawed for 2 hours outside refrigerator. Two rolls of phyllo sheets come in a package, so only thaw 1 roll. 1/2 cup extra virgin olive oil or as needed
Filling: In a medium bowl, stir together spinach, feta, ricotta, eggs, salt and pepper. Cover and refrigerate until ready to use. Heat butter and olive oil in a medium skillet, then sauté mushrooms and onions over medium heat, stirring often, until well browned, about 20 minutes.
Assembly: Preheat oven to 350°. Brush the bottom and sides of the 9" x 9" baking dish with olive oil. Gently unfold the phyllo sheets and lay 2 sheets over the bottom of the baking dish - the excess phyllo will drape over the edges - do not trim. If sheets tear slightly, it's fine. Tuck sheets snuggly into the bottom of the dish then brush top sheet generously with olive oil (keep the sheets together in pairs). Repeat this same step two more times, alternating the direction you lay the sheets each time (using a total of 6 sheets). Spread the mushroom and onion mixture over the phyllo. Spread the cheese filling over the mushrooms. To create the top layer, repeat as you did with the bottom layers by layering two sheets of phyllo at a time, brushing with olive oil after each layer. Fold the excess phyllo sheets which are draped over the edges inward onto the top and generously brush with olive oil. Bake for 45-50 minutes or until golden brown.