Preheat oven to 375°F (190°C). Grease a 9×9-inch baking dish (or similar size).
In a skillet, heat olive oil over medium heat. Sauté the onion until soft, about 3–4 minutes. Add garlic and cook 1 minute more.
Stir in the spinach and cook briefly to remove any excess moisture. Remove from heat.
In a large bowl, whisk together eggs, milk, salt, pepper, and nutmeg.
Stir in the spinach mixture and feta cheese (plus mozzarella if using).
Pour mixture into the greased baking dish and smooth out evenly.
Bake uncovered for 30–35 minutes, or until the eggs are set and the top is golden.
Let cool for 5 minutes before slicing into squares and serving warm.

✨ Serving Idea: Pair with fresh fruit or a slice of crusty bread for a complete breakfast.

Fresh vs. Frozen Spinach: If using fresh spinach, lightly sauté until wilted and drain well. If using frozen, make sure to squeeze out as much liquid as possible to avoid a watery casserole.
Cheese Options: Feta adds tang, but you can swap in goat cheese for creaminess, or add mozzarella/cheddar for extra meltiness.
Make Ahead: This casserole can be assembled the night before. Just cover and refrigerate, then bake in the morning (add 5–10 extra minutes baking time if cold).
Storage: Leftovers store well in the fridge for up to 4 days. Reheat individual squares in the microwave or oven.
Freezer Friendly: Bake, cool completely, cut into portions, and freeze. Reheat directly from frozen at 350°F (175°C) until warmed through.
Add-Ins: You can add cooked bacon, ham, or sautéed mushrooms for extra flavor and protein.