Preheat oven to 350°F (175°C) and lightly grease a 9-inch baking dish.

In a skillet over medium heat, sauté chopped onion in olive oil until translucent (about 5 minutes). Add minced garlic and fresh spinach; cook until spinach is wilted.

In a bowl, whisk together eggs, milk, salt, and pepper until frothy.

Fold in the sautéed spinach mixture and crumbled feta into the egg mixture.

Pour the mixture into the prepared baking dish and bake for 30-35 minutes until puffed and golden.

Allow cooling slightly before slicing; serve warm.