01 - Preheat oven to 375°F.
02 - In an ovenproof skillet, heat olive oil over medium heat. Add the diced onion and sauté 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 2–3 minutes.
04 - In a mixing bowl, whisk together the eggs, milk, salt, pepper, and dried oregano and nutmeg if using. Stir in half of the crumbled feta cheese.
05 - Pour the egg mixture evenly over the spinach mixture in the skillet. Gently shake the skillet to distribute the ingredients evenly.
06 - Sprinkle the remaining feta cheese evenly over the top of the frittata.
07 - Cook on the stove over low heat for 2–3 minutes until the edges begin to set.
08 - Transfer the skillet to the preheated oven and bake for 10–12 minutes, or until the center is set and the top is lightly golden.
09 - Let the frittata rest for a few minutes, then slice into wedges and serve warm or at room temperature.