Whisk the reserved pineapple juice, sugar, cornstarch and salt in a saucepan and cook over medium heat until bubbling and thick.
Stir in the drained pineapple, lemon juice and butter, cook for 2 minutes, and cool for 20 minutes while heating the oven to 400°F.
Fit one rolled dough disk into a 9-inch plate and add the cooled pineapple filling.
Cover with the second rolled crust, trim and crimp, cut steam vents, and brush with egg wash.
Bake for 20 minutes, lower the oven to 375°F, and bake 25 to 30 minutes more until the crust is deep golden and the filling bubbles.
Cool for 3 hours before slicing.