Preheat your oven to 375°F (190°C).

In an oven-safe skillet, heat the olive oil over medium heat. Add the finely chopped onion and sauté until translucent, about 3-5 minutes. Then, add the minced garlic and cook for an additional minute until fragrant.

Stir in the chopped spinach and cook until wilted, about 2-3 minutes.

In a large bowl, whisk together the eggs, milk, salt, and pepper. Optionally, add fresh herbs.

Pour the egg mixture over the sautéed spinach and onion in the skillet. Stir gently to combine and sprinkle crumbled feta cheese on top.

Cook on the stovetop for about 3-4 minutes until the edges begin to set.

Transfer the skillet to the preheated oven and bake for about 20-25 minutes until set in the center and lightly golden on top.