Preheat your oven to 180°C (350°F). Roll out the chilled pie crust on a lightly floured surface to about 0.5 cm thick, then gently fit it into your 9-inch pie dish. Prick the bottom with a fork to prevent bubbling, then place in the freezer for 10 minutes to relax the dough.
Remove the crust from the freezer and bake it blind: line the crust with parchment paper and fill with pie weights or dried beans. Bake for about 10 minutes until the edges just start to turn golden, then remove the weights and parchment, and bake an additional 5 minutes to set the crust.
While the crust bakes, heat oil in a skillet over medium heat. Add the onions and cook slowly, stirring occasionally, until they are golden brown and caramelized, about 15 minutes. Transfer to a bowl and set aside.
In the same skillet, add a splash more oil if needed, then sauté the chopped spinach until wilted and fragrant, about 2-3 minutes. Transfer the spinach to a fine mesh sieve, squeeze out excess moisture, and let cool slightly.
In a large bowl, whisk together the eggs until frothy and light. Mix in the heavy cream, grated cheese, nutmeg, salt, and pepper, whisking until well combined.
Fold the sautéed spinach and caramelized onions into the custard mixture, ensuring everything is evenly distributed.
Pour the filling into the pre-baked crust, spreading it out evenly. Place the pie dish on a baking sheet to catch any overflow.
Bake the quiche at 180°C (350°F) for 35-40 minutes, until the edges are golden and the center is just set but still slightly wobbly when gently shaken. Cover the edges with foil if they brown too quickly.
Remove the quiche from the oven and let it rest on a wire rack for about 10 minutes. This helps the filling set and makes slicing easier.
Slice the quiche and serve warm or at room temperature, enjoying the flaky crust, tender greens, and melted cheese in every bite.
