01 - Preheat oven to 375°F.
02 - Heat olive oil in an ovenproof skillet 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, and optional spices. Stir in half of the crumbled feta cheese.
05 - Pour the egg mixture evenly over the spinach mixture in the skillet. Gently shake the pan to distribute everything evenly.
06 - Sprinkle the remaining feta cheese on top of the egg mixture.
07 - Cook on the stove over low heat for 2 to 3 minutes until the edges begin to set.
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 - Let the frittata rest for a few minutes, then slice into wedges and serve warm or at room temperature.