Preheat oven to 400 degrees F. Place a 9-inch pie dish in the oven.
In a large bowl, whisk together the eggs, yogurt, salt, pepper and onion powder. Stir in the cheese and the spinach.
Carefully, using oven mitts, remove the baking dish from the hot oven. Spray with nonstick spray. Transfer the egg mixture to the baking dish. Bake 30-35 minutes, until golden brown and puffy and a knife inserted in center comes out clean.
Allow to cool 5 minutes in pan, then invert onto a platter and from the platter to a cooling rack. Cool 10 more minutes before serving - frittatas taste better at room temperature than when hot.