Mix the flour and salt in a bowl, then cut in the cold butter using your fingers or a pastry cutter until the mixture resembles coarse crumbs.
Add ice water gradually, a tablespoon at a time, mixing until the dough begins to come together. Form into a disk, wrap in plastic, and chill for 15 minutes.
Preheat your oven to 190°C (375°F). Roll out the chilled dough on a floured surface into a 12-inch circle, then carefully transfer it to your tart pan or pie dish. Press into the edges and trim excess.
Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes until the edges start to turn golden. Remove the weights and parchment, then bake for another 5 minutes to crisp the base.
Trim the asparagus ends and blanch the spears in boiling water for about 2 minutes until just tender. Transfer immediately to ice water to stop the cooking, then drain and chop into 1-inch pieces.
Wilt the spinach in a hot skillet until just tender, then squeeze out excess moisture and chop roughly. This prevents a soggy crust.
Whisk the eggs and heavy cream together until smooth and slightly frothy. Stir in the grated Gruyère cheese, then fold in the chopped asparagus and spinach.
Pour the custard mixture into the pre-baked crust and smooth the surface with a spatula. Place in the oven and bake for 35-40 minutes, until the filling is set and lightly golden on top.
Check for doneness by gently jiggling the center; it should wobble slightly but not liquid. The top should be golden and fragrant with herbs.
Allow the quiche to rest for about 10 minutes outside the oven; this helps it set and makes slicing easier. Garnish with fresh thyme or chives if desired.
Slice into wedges and serve warm or at room temperature, enjoying the tender greens, creamy custard, and flaky crust in every bite.
