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 in about 1/4 cup (60 mL) cold water just until moistened, but still slightly crumbly. The mixture should hold together when pinched. Stir in up to 1 tbsp (15 mL) additional water, if needed. Pat into a ball. Divide in half; shape into 2 disks. Place each in plastic wrap. Chill at least 1 hr.
Preheat oven to 190°C (375°F). Roll out 1 disk to 1/8-in. (3 mm) thick circle large enough to line the bottom, sides and rim of a 9-in. (23 cm) pie plate. Trim off excess pastry. Prick bottom with a fork a few times. Freeze pie shell 10 min.
Meanwhile, in large bowl, toss together rhubarb, strawberries, sugar, cornstarch, lemon zest, juice and vanilla. Take pie shell from freezer. Pour filling into pie shell (including juices in bowl). Cut reserved 2 tbsp (30 mL) butter cut into small pieces. Dot filling with the butter.
Roll second pastry disk into 1/8-in. (3 mm) thick circle at least 2 in. (5 cm) wider than the pie. Cut into 8 strips about 1 in. (2.5 cm) wide. Arrange over filling and weave into a lattice top. trim off excess pastry at edges. Use a fork to press tines into the edge or crimp with fingers. (As an easy alternative to a lattice top, use a 2-in. (5 cm) diameter cookie-cutter, such as a star shape, to make cut-outs in rolled pastry and arrange them to partially cover the filling.)
Fork-beat egg with 1 tbsp (15 mL) water. Brush pastry with egg wash. Place pie on baking sheet. Bake 1 hr. to 1 hr. 5 min. until filling is bubbling in the centre and pastry is golden. Transfer pie to wire rack to cool completely. Can be made up to 1 day in advance.