To make pastry, in large bowl, whisk flour with salt. Reserve 2 tbsp (30 mL) butter. Cut remaining butter into flour mixture with pastry-blender or rub with fingertips until mixture resembles coarse meal. Use fork to stir approx. ¼ cup (60 mL) cold water into flour mixture until moistened, but slightly crumbly. Dough should come together when pinched. Stir in up to 1 tbsp (15 mL) water, if needed. Form dough into ball. Divide in half and shape into 2 disks. Wrap each disk in plastic wrap and refrigerate at least 1 hr.
Preheat oven to 375ºF (190ºC). Roll out one disk of dough to line bottom, sides and rim of 9-in. (23-cm) pie plate. Trim off excess pastry. Use fork tines to press decorative pattern onto edge or crimp edge with fingers. Freeze pie shell 10 min.
Meanwhile, roll remaining dough to ⅛-in. (3-mm) thickness. Use 2-in. (5-cm) diameter star cookie-cutter to cut out about 24 stars, re-rolling dough scraps once. Place cut-outs on parchment-lined baking sheet; freeze about 5 min. Meanwhile, in large bowl, toss rhubarb, strawberries, sugar, cornstarch, lemon zest, lemon juice and vanilla. Scrape mixture into pie shell (including juices in bowl).
Dot filling with reserved 2 tbsp (30 mL) butter cut into small pieces. Fork-beat egg with 1 tbsp (15 mL) water. Brush tops of chilled cut-outs and pie shell edge with egg wash. Starting from edge of pie, lay star cut-outs in circular pattern on filling, with tips of stars touching. Place pie on parchment-lined baking sheet. Bake 60 to 65 min., until filling is bubbling in the centre and crust is golden brown. Transfer to wire rack to cool completely. Can be made up to 1 day in advance.