01 - Preheat oven to 375°F.
02 - In an ovenproof skillet, heat olive oil over medium heat. Add the onion and sauté for 3 minutes until softened. Add garlic and cook for another 30 seconds until fragrant.
03 - Add chopped spinach to the skillet and cook until wilted, about 2 to 3 minutes.
04 - In a mixing bowl, whisk together eggs, milk, salt, pepper, oregano, and nutmeg. 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.
06 - Sprinkle the remaining feta cheese evenly on top.
07 - Cook on the stove over low heat for 2 to 3 minutes until the edges begin to set.
08 - Transfer 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 - Let the frittata rest for a few minutes, then slice into wedges and serve warm or at room temperature.