Preheat the oven to 350°F (175°C). Lightly oil a 9-inch pie dish or similar baking pan.

Heat the olive oil in a skillet over medium heat. Add the chopped onion and sauté for 6–8 minutes until soft and beginning to brown.

Add the chopped spinach to the pan. Cook for 1–2 minutes until wilted. Remove from heat and squeeze to remove excess liquid.

In a large bowl, whisk together the eggs, milk, salt, pepper, and nutmeg until smooth.

Stir the crumbled feta into the egg mixture.

Fold the squeezed spinach and sautéed onions into the bowl and mix until evenly distributed.

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

Bake on the center rack for 30–35 minutes, or until the edges are set and the center jiggles only slightly. The internal temperature should reach about 160°F.

Let the quiche cool for 8–10 minutes before slicing to help it set.