In a food processor pulse flour, salt and sugar until combined.
Add the cold, cubed butter and pulse until mixture resembles a course meal.
Add water and egg yolk. Pulse until dough forms a cohesive mass.
Press dough into a ball and chill for an hour
Rhubarb Strawberry Filling
Toss all ingredients (except the 2 egg yolks and 1 tablespoon of cream) together in a bowl. Cover with plastic wrap and let sit for 30 minutes.
When you're ready to assemble, have a sheet pan lined with a silicone baking mat ready. Roll out the dough to a 12" circle. Place on the silicone mat/cookie sheet. The edges will be rough.
Place the fruit in the middle of the tart, leaving about 2 inches of tart dough around the mound in the middle.
Begin folding the dough around the tart to wrap the fruit in the pastry. Chill under refrigeration for 10 minutes.
While the tart is chilling, mix together the two egg yolks and 1 Tablespoon of heavy cream to create your egg wash.
Preheat your oven to 350 degrees and place a rack in the middle of the oven. Generously brush the outside of the pastry with the egg was and sprinkle with the remaining granulated sugar.
Bake for 45 minutes to 1 hour and 15 minutes. This is all dependent on your oven and how dark you like your pastry to be. I tend to like mine on the darker side with the crust turns a deep golden brown and the fruit begins to turn black on the tips. Let sit for at least 15 minutes before you cut into it. It is delicious by itself or with caramel, vanilla ice cream or a dollop of mascarpone cheese.
The tart can be chilled completely and stored under refrigeration for up to one week.