01 - Preheat oven to 375°F.
02 - Heat olive oil in an ovenproof skillet over medium heat. Add the diced onion and saute for 3 minutes until softened. Add the minced garlic and cook for another 30 seconds until fragrant.
03 - Add the chopped spinach to the skillet and cook, stirring occasionally, until wilted, about 2 to 3 minutes.
04 - In a mixing bowl, whisk together the eggs, milk, salt, black pepper, oregano, and nutmeg until well combined. Fold in half of the crumbled feta cheese.
05 - Pour the egg mixture evenly over the sauteed spinach in the skillet. Gently shake the pan to distribute the ingredients in an even layer.
06 - Sprinkle the remaining crumbled feta cheese evenly over the top of the frittata.
07 - Cook the frittata on the stovetop over low heat for 2 to 3 minutes until the edges begin to set and pull away slightly from the sides of the skillet.
08 - Transfer the skillet to the preheated oven and bake for 10 to 12 minutes, or until the center is set and the top is lightly golden.
09 - Remove from the oven and let the frittata rest for a few minutes. Slice into wedges and serve warm or at room temperature.