Pre-heat oven to 400°F. Roll the prepared pie crust out and place in a 9-inch pie plate and flute the edges. Set aside.
In a non stick frying pan, heat the oil over medium heat.
Add the onion, and cook until softened and transluscent, around 5 minutes. Add the garlic and cook until fragrant, 1 minute.
Add the zucchini and cook, stirring often, for 5-10 minutes, or until cooked through.
Remove from heat and allow to cool for a few minutes.
In a large bowl, beat the eggs. Add the ricotta and milk, and mix until smooth. Stir in the salt, garlic powder, oregano, pepper, and ½ cup of mozzarella cheese.
Spread out the zucchini/onion/garlic mixture into the prepared pie crust (you may wish to set aside half of the zucchini slices to arrange over the top as in the picture). Pour the egg/ricotta mixture over top. Arrange zucchini slices around the edge of the quiche (if desired), sprinkle the top of the quiche with remaining mozzarella, and with the walnuts.
Bake for 30-40 minutes, until top is slightly golden, and quiche is cooked through the middle.
Cool for 10 minutes before serving.