Preheat your oven to 375°F. Grease a 9-inch pie dish or round baking dish to prevent sticking.

In a small skillet, heat the olive oil over medium heat. Add the diced onion and sauté for about 3 minutes, until softened. Then add the minced garlic and cook for an additional 30 seconds. Remove from heat and allow to cool slightly.

In a large bowl, whisk together the eggs, whole milk, heavy cream, salt, pepper, and ground nutmeg until combined and smooth.

Stir in the drained spinach, sautéed onion and garlic, crumbled feta cheese, and shredded mozzarella cheese until well mixed.

Pour the mixture into the prepared baking dish and spread it out evenly. Bake in the preheated oven for about 35-40 minutes, until the center is set and the top is lightly golden. You can check for doneness by inserting a knife into the center; it should come out clean.

Let the quiche rest for 10 minutes after baking to firm up for easier slicing. Cut into wedges and serve warm or at room temperature.