Preheat oven to 375 degrees. Spray two standard muffins pan with cooking spray.

In a large skillet over medium high heat, heat the oil. Add the onions and cook until tender, about 5 minutes. Add the spinach and cook, stirring, until wilted, about 3 minutes. Let cool.

In a large mixing bowl, whisk together the eggs, ricotta cheese, milk, flour, baking powder, oregano, salt, and pepper. Add the feta cheese, tomatoes, olives, and spinach mixture. Stir well to combine.

Spoon the mixture into the prepared muffin pans and bake for 30 minutes. Can be served hot, warm, or cold. Store in the refrigerator in an airtight container or food storage bag.