Heat the oven to 350°F. Medium-high heat a 10-inch cast iron skillet with oil. Add spinach and toss regularly until wilted, 2–3 minutes.
The Greek frittata should be golden brown and cooked through after 35–45 minutes on the centre rack of the preheated oven.
Let the frittata cool for 10 minutes before slicing and serving. Sitting the frittata lets it set and makes it simpler to slice.