Preheat your oven to 180°C (350°F). Roll out your pie crust and carefully line a quiche or tart pan, pressing it gently into the edges. Trim any excess dough, then prick the base all over with a fork—this helps prevent it from puffing up in the oven. Place the crust in the fridge and let it chill for 15–20 minutes so it holds its shape during baking.
Once chilled, line the crust with parchment paper and fill it with baking weights or dried beans. Blind bake for about 15 minutes, then remove the weights and parchment and return it to the oven for another 5–10 minutes, until the base is lightly golden and set.
While the crust is baking, heat a bit of oil in a pan over medium heat and cook the onions until soft and slightly sweet. Add the spinach and cook it down until fully wilted. Let the mixture cool slightly, then squeeze out as much excess moisture as possible—this step is key to keeping the quiche from becoming watery.
In a mixing bowl, whisk together the eggs and cream until smooth. Season with salt, pepper, and a small pinch of nutmeg. Add the cooked spinach and onions to the egg mixture and stir until everything is evenly combined.
Pour the filling into the pre-baked crust, then crumble the feta cheese evenly over the top so it stays slightly defined as it bakes.
Place the quiche in the oven and bake for 35–40 minutes, or until the edges are set and the center still has a slight wobble. Remove from the oven and let it rest for 10–15 minutes before slicing—this allows the custard to fully set and makes serving much easier.