Mix the flour and salt together in a large bowl.
Using a pastry blender or fork, cut in cold butter until you have pea-sized pieces.
Sprinkle ice cold water over the mixture one tbsp at a time and work the dough with your hands. Stop adding water when the dough starts to form large clumps. Continue working with your hands until the flour is just incorporated and the dough comes together. Do not overwork, you should still be able to see specks of butter.
Divide in half, wrap in plastic wrap and chill in the refrigerator for at least 30 minutes.
In the meantime, prepare the filling by chopping strawberries and rhubarb. Add to a large bowl with sugar and cornstarch, stir to coat evenly.
Preheat oven to 375°F.
Place chilled dough on a piece of parchment paper and roll into roughly a 10" circle. Don't worry if the edges are ragged or it's not a perfect circle, but aim for an even thickness.
Add approximately half of the strawberry filling to the center of the dough, leaving about 2" all the way around. Fold the border over the filling, overlapping where needed and tucking under any rough edges. Gently use a spatula to help you if the border sticks to the parchment (don't worry about the bottom, it won't stick once it bakes).
Brush the edge of the dough with a beaten egg. Optional: sprinkle with coarse sugar for some extra sparkle and a little crunch.
Transfer the parchment paper to a baking tray and bake 30-40 minutes, until crust is golden brown and the filling is bubbling.
Repeat steps 7-10 with remaining dough and filling. Serve warm or cold. Delicious on its own or topped with ice cream or fresh whipped cream.