Preheat your oven to 375°F (190°C).

In a skillet over medium heat, add olive oil. Once hot, add diced onion and minced garlic, sautéing until the onion becomes translucent, about 3-4 minutes.

Toss in the chopped spinach and cook for another 2-3 minutes until it wilts down. Season with salt, pepper, and nutmeg if using. Remove from heat and set aside to cool slightly.

In a large mixing bowl, crack the eggs and whisk them until well-combined. Stir in the sautéed spinach mixture and shredded mozzarella cheese.

Grease a baking dish with a little olive oil or butter. Pour the egg mixture into the dish, distributing the spinach and cheese evenly.

Place in the preheated oven and bake for about 25-30 minutes, or until the eggs are set and lightly golden on top.

Allow it to cool slightly before slicing. Serve warm, garnished with cherry tomatoes or fresh herbs if desired.