Preheat the oven to 375°F (190°C).
In a large bowl, whisk together the eggs, milk, salt, and pepper. Set aside.
In a 10-inch oven-safe skillet, heat the olive oil over medium heat. Add the diced onion and minced garlic, and cook until the onion is soft and translucent, about 5 minutes.
Add the chopped spinach to the skillet and cook until wilted, about 2-3 minutes.
Pour the egg mixture over the spinach and onion mixture. Sprinkle the crumbled feta cheese over the top.
Place the skillet in the preheated oven and bake for 20-25 minutes, or until the frittata is set in the center and the edges are golden brown.
Remove the skillet from the oven and let it cool for a few minutes before slicing and serving.