Combine flour, salt, and sugar. Cut in cold butter until mixture resembles coarse crumbs with pea-sized bits. Add ice water one tablespoon at a time until dough just comes together. Form a disk, wrap, and chill 30 minutes.
Toss sliced nectarines with sugar, cornstarch, lemon juice, lemon zest, cinnamon (if using), and salt. Let sit 10 minutes to draw out juices.
Preheat oven to 400°F (200°C). Roll chilled dough into a 12-inch circle and transfer to a 9-inch pie dish. Trim and crimp edges. Add nectarine filling to crust.
Scatter crumble topping over the filling or brush crust edge with beaten egg and sprinkle coarse sugar. Bake at 400°F for 15 minutes, then reduce to 375°F and bake 30–35 minutes until bubbling and golden. Tent with foil if edges brown too quickly.
Cool at least 2 hours to allow filling to set. Serve warm or at room temperature with ice cream if desired.
Notes
If nectarines are very juicy, add an extra tablespoon of cornstarch.
Chill the crust to ensure flakiness.
Tent with foil if edges brown before the filling is done.
Store covered at room temperature for up to 2 days or refrigerated for up to 4 days.