Preheat oven to 400°F.

In a medium bowl, whisk together eggs, milk, oregano, kosher salt, and fresh ground pepper. Whisk in the Parmesan and feta cheese.

In a 10-inch oven-safe skillet, heat the olive oil over medium heat. Sauté the spinach leaves, stirring until wilted, 2 to 3 minutes, spreading it around as you cook so that you spread oil up the sides of the pan. Add the garlic powder and 2 pinches kosher salt and sauté for another minute.

Pour the egg mixture into the skillet. If necessary, use a fork to rearrange the spinach in an even layer so you can see if on the top. Sprinkle the top with a little more feta cheese. Cook for about 5 to 7 minutes over medium heat until the bottom is set and lightly browned, checking for doneness by pulling back the side with a spatula.

Place the skillet in the oven and bake 10 to 12 minutes until the top is puffed, set, and slightly browned. Let rest for 10 minutes, then cut into wedges and serve warm, with sour cream or crème fraiche if desired. Leftovers save well: refrigerate for up to 5 days and serve them cold, room temperature, or warmed.