Saute sliced onion in some olive oil in a skillet and add the defrosted and drained spinach to the pan. Cook for a few minutes until onion is translucent. Remove from the stove so the onion spinach mixture cools.
Whisk eggs and cheese together and when the spinach is cool enough, add to the eggs and cheese mixture. Pour into a 9" greased pie dish and bake in a 350F oven for 25-30 minutes. Let rest a few minutes before serving.