Combine rhubarb with strawberries in a large mixing bowl.
Add sugar, brown sugar, salt, corn starch and vanilla.
Stir gently until all fruit is coated.
Preheat oven to 390 F
Remove the chilled crust dough from the frig after at least one hour.
Place a long piece of parchment paper on the workspace and put one of the dough discs on top and then place another long piece of parchment paper on top of the dough.
Using a rolling pin, roll out the dough into a circle that will fit your pie pan with a little extra.
Peel off the top parchment paper layer. Be very careful and sprinkle the top of the dough with flour and place the paper back on top and flip it over.
With care, peel of the new top piece of parchment paper and carefully place the dough in an unbuttered pie baking dish.
Press the crust dough into the baking dish and trim any excess dough off of the sides.
Drain the liquids from the rhubarb and strawberry mixture.
Scoop the fruit onto the pie crust dough.
With the second circular disc of dough, use the same methods above to roll out the top crust layer.
Cut strips approximately ½" wide and then weave into a lattice-top crust.
Brush the top of the crust with the egg wash. Sprinkle very generously with sugar.
Bake for 20 minutes in the lowest rack of the oven at 390 F.
Keep an eye on the crust to prevent burning; place aluminum foil on the top as soon as it begins to look like it is getting too dark and not a golden brown.
Reduce the oven temperature after 20 minutes to 350 F and bake for another 40 minutes when the juices bubble and the crust is golden brown.
Prepare the Mascarpone Cream
Combine the mascarpone cheese with cream, sugar and vanilla; mix until thickened.
Remove pie from oven and allow to cool.
Top with the sweet mascarpone cream.