Combine the diced rhubarb and strawberry in a bowl, add the lemon juice, sugar and tapioca or corn starch. Mix well and let rest for at least 30 minutes, stirring every once in a while.
Sweet tart dough preparation
Combine the flour, sugar and salt in a food processor and mix.
Add the butter pieces, in batches, and pulse until mixed.
Add the egg and the water, 1 tablespoon at a time, until clumps of dough start to form.
Remove the dough from the food processor and knead it gently.
Mold the dough into a flattened thick disc, and chill in the refrigerator for at least 30 minutes.
On a floured sheet of wax paper, roll out the dough into a thin sheet that is a couple inches larger than the tart pan you plan to use – it’s okay if it extends beyond the edges of sheet of wax paper.
For the classic tart
Carefully flip the wax paper and the sheet of rolled pastry dough on to a lightly greased tart pan. Gently press the dough with your fingers onto the mold, start at the center and move outward. Use a rolling pin to roll over the edges and remove any excess dough from the edges. If you want you can leave a ½ - 1 inch to wrap around the edges of the pan – this will help prevent the crust from shrinking.
Prick the tart shell with a fork and let it rest in the refrigerator for at least 30 minutes or until ready to use.
For the rustic tart or galette
Carefully transfer the rolled out tart dough onto a flat baking pan lined with parchment paper.
Use a slotted spoon to transfer the rhubarb and strawberry filling to the center of tart. Try to avoid adding any of the liquid at this moment, but save it for later. Leave the outer 2 to 4 inches of tart dough free of any fruit filling. Next fold the edges of the tart dough over the filling, it won’t cover the entire filling and is mainly meant to contain and keep the filling inside.
Refrigerate the rustic tart for at least 30 minutes.
Baking the tart
Pre-heat the oven to 400F.
If making the classic tart, then pour the rhubarb strawberry filling into the tart and place it in the pre-heated oven.
If making the rustic tart, brush the edges with the egg wash and sprinkle with some sugar. At this point, you can also use a regular spoon to add the liquid from the fruit filling back into the tart. Place the tart in the pre-heated oven.
Bake the tart for about 35-40 minutes or until the crust is golden.