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