Preheat your oven to 190°C (375°F). Place the pre-baked pie crust on a baking sheet and set aside.
Heat a skillet over medium heat and add a splash of olive oil. Once shimmering, add the chopped spinach and sauté for about 2-3 minutes until wilted and vibrant green. Remove from heat and let it cool slightly.
In a mixing bowl, whisk together the eggs, heavy cream, salt, pepper, and a pinch of freshly grated nutmeg until smooth and slightly frothy.
Stir the cooled spinach into the egg mixture, then fold in the grated cheddar and Gruyère cheeses, ensuring an even distribution.
Pour the filling into the pre-baked pie crust, spreading it out evenly with a spoon or spatula.
Bake the quiche in the oven for 35-40 minutes, until the top is golden brown and the filling is set with a slight jiggle in the center.
Remove the quiche from the oven and allow it to rest on a wire rack for at least 10 minutes. This helps the filling firm up for cleaner slices and enhances flavor.
Slice the quiche with a sharp knife, serving warm or at room temperature. The crust should be crisp, and the filling creamy with melted, bubbly cheese on top.