Preheat oven to 375ºF and line baking sheet with parchment or a Silpat.
Place rhubarb in a large bowl and add sugar, cornstarch, salt, and rose water. Mix well until combined.
Roll out pie dough to an 11" disc and place on the lined baking sheet. Place rhubarb in the middle of the pie crust leaving a 2" border. Fold dough over and pinch to enclose the rhubarb, leaving the center open.
Brush crust with egg wash and sprinkle with coarse sugar.
Bake for 35-40mins, until crust is golden and juices are bubbling.
Cool on sheet for 10 mins. Slide off with parchment onto wire rack to cool completely.