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

In a skillet, heat 1 tablespoon of olive oil over medium heat, add the chopped onion, and cook until soft and translucent, about 6–8 minutes. Let cool slightly.

Add the chopped spinach to the skillet for 1–2 minutes until just wilted. Remove from heat and squeeze out excess moisture with a clean towel or spoon.

In a bowl, whisk together the eggs, milk, salt, black pepper, and nutmeg until well combined.

Stir the sautéed onion and spinach into the egg mixture and fold in the crumbled feta cheese.

Pour the mixture into the prepared dish and smooth the top.

Bake for 30–40 minutes, checking at 30 minutes; the center should be set but slightly springy.

Allow the quiche to rest for 10–15 minutes before slicing and serving.