In a large mixing bowl, mix the rhubarb, 1 cup of granulated sugar, 3 tablespoons of flour, pinch of salt and the lemon zest. Pour into the prepared pie crust.
In a small mixing bowl, combine the remaining sugar, flour and softened butter. Mix with a fork until crumbly; then spread on top of the rhubarb mixture.
Place the pie on a baking sheet to catch any juices from the rhubarb that boil over. Cover the crust with a pie shield or foil to prevent the crust from burning.
Bake the pie at 450℉ for 10 minutes; reduce heat to 350℉ and bake for 40-45 minutes until the crumble mixture begins to turn golden brown.
Allow pie to cool completely before serving.
Notes
Store baked pie up to 72 hours; store refrigerated
If using frozen rhubarb, allow to thaw completely and drain off the extra liquid before using.
Do not decrease the sugar in this recipe; the rhubarb requires a lot of sugar to offset its tartness.