Preheat the oven to 375°F.
In a large bowl, whisk together the eggs, milk, salt, and pepper until well combined.
Heat the olive oil in a 10-inch oven-safe skillet over medium heat. Add the onion and garlic and cook until softened, about 5 minutes.
Add the spinach to the skillet and cook until wilted, about 2 minutes.
Pour the egg mixture into the skillet and stir gently to distribute the spinach and onion evenly.
Sprinkle the feta cheese over the top of the egg mixture.
Transfer the skillet to the oven and bake for 15-20 minutes, or until the frittata is set and the top is golden brown.
Remove the skillet from the oven and let the frittata cool for a few minutes.
Sprinkle the chopped parsley over the top of the frittata.
Slice the frittata into wedges and serve warm or at room temperature. Enjoy!