In a large bowl combine flour and sugars. Add butter cubes, rub in with your fingers until the mixture looks like fine crumbs. Add the egg and combine everything to a dough. Place in plastic wrap and let it cool in the fridge for about 30 minutes.
Preheat oven to 180°C.
When the dough is ready roll it out on a lightly floured surface (or directly on the parchment paper) to about 1/4 inch thickness. Transfer to a baking sheet lined with parchment paper.
Combine the rhubarb, strawberries, brown sugar, cornstarch and vanille bean paste
Spread the filling evenly onto dough, leaving a 3 inch border around the edges. Fold edges over, overlapping slightly. Brush dough with egg wash and sprinkle with brown sugar.
Bake for 40-50 minutes until golden brown. Let cool slightly and serve with some whipped cream or ice cream.