Make the pie dough - sift together the flour and salt. Rub the butter/shortening in until it resembles breadcrumbs for a fine crust, or small peas for a flaky crust.
Mix the egg, water, and vinegar together, and pour into the flour mixture. Mix together with a spoon, then turn out onto the countertop and press together. Divide into two flattened disks, and wrap in cling film.
Refrigerate for 30 minutes.
Roll out one disk of dough and line a 9" pie dish. Decorate as desired.
Preheat the oven to 175°C convection (350°F).
Make the filling: mix together the strawberries, rhubarb, lemon juice, sugar, and flour.
If using the tapioca, sprinkle on the bottom of the pie crust before adding in the strawberries and rhubarb.
Make the crumble topping: mix together everything but the butter. Then rub in the butter until it forms a crumble. Sprinkle on top of the pie filling.
Make an egg wash by whisking a whole egg, then brush on the pie crust (but not the crumble).
Bake for about 40-50 minutes in the preheated oven, until the crust is golden brown, and the filling has bubbled up through the crumble slightly.
Allow to cool a bit before serving with vanilla ice cream! Enjoy!