Preheat oven to 375°F (190°C).
Whisk eggs, milk, salt, pepper, and herbs in a bowl.
Heat olive oil in an oven-safe skillet over medium heat.
Sauté onion until soft, then add garlic and cook briefly.
Add spinach and cook until wilted.
Stir in chicken and spread evenly.
Pour egg mixture over filling and gently stir.
Sprinkle feta cheese evenly on top.
Cook on stovetop for 3–4 minutes until edges set.
Transfer skillet to oven and bake for 18–22 minutes until set.