Line a large rimmed baking sheet with parchment; set aside.
Pulse together flour and salt in a food processor. Add butter and shortening and pulse dough to hazelnut-sized pieces. Pulse in water and vinegar until dough begins to come together. Turn the dough out onto plastic wrap, wrap and press the dough into a 2.5 centimeter thick disk. Refrigerate for 45 minutes.
Preheat oven to 400°F (205°C)
Combine strawberries, rhubarb, orange zest, vanilla, granulated sugar, cornstarch, cinnamon and grated nutmeg in a large bowl.
On a lightly floured surface, roll the dough into a six millimeter thick and 25 centimeter round piece, and transfer to prepared sheet. Spoon fruit mixture into the centre of the dough, leaving a five centimeter border. Fold the edges of the galette up over the fruit to form a crust.
Whisk together the egg and milk in a small bowl. Brush mixture onto crust and then sprinkle crust with turbinado sugar.
Bake until crust is golden and fruit is tender, about 35 to 40 minutes. Cool to room temperature before cutting to serve.