Preheat the oven to 350 F.
Brush a 9"x13" baking pan liberally with olive oil.
In a large saute pan over medium high heat, add 1 tablespoon of garlic olive oil. Add the diced onion and saute until translucent. Add the greens and saute until cooked down, about 3 additional minutes. Add the feta and ricotta to the greens and mix thoroughly. Add 1 teaspoon cornstarch to 2 tablespoons of water and add to the greens. Continue cooking over medium high heat until the mixture thickens and season with salt and fresh ground pepper to taste. Allow the mixture to cool until just barely warm.
Take the thawed phyllo out of the refrigerator. Unroll the sheets and cover with a clean barely dampened towel.
Liberally brush three sheets or spray three sheets with garlic infused olive oil and layer one on top of another, lining the bottom and part way up the sides of the baking pan. Add 1/3 of the spinach mixture and spread evenly over the phyllo. Continue brushing each sheet with olive oil and layering three sheets at a time followed by two more layers of spinach and ending with three sheets of phyllo on top.
Bake for 35-40 minutes until the top is deep golden brown and crisp. Allow to cool slightly and cut into squares.