Place a rack in the middle of the oven and preheat to 375°. Defrost piecrust for 15 minutes; then prick bottom and sides generously with a fork. Place on a rimmed baking sheet and bake until beginning to turn golden, 8 to 10 minutes. Remove from oven and let cool slightly. Brush bottom and sides of crust with egg yolk and water mixture, then return to the oven and bake until egg wash is dry, about 5 minutes more. Transfer to a rack and let cool.
Drain rhubarb in a fine strainer (reserve can liquid for another use). Transfer fruit to a large bowl and toss with sugar, cornstarch, lemon juice, lemon zest and salt.
Pour rhubarb mixture into the piecrust and sprinkle with crumble topping. Bake until topping is browned, 40-45 minutes. (If topping or crust begins to brown too quickly, tent with foil.) Cool completely before serving.
For the crumble topping:
Mix flour, brown sugar and salt in a small bowl. With your hands, work in butter, squeezing mixture until large, moist crumbs form.