Mix the all-purpose flour, almond flour, kosher salt and powdered sugar in a large bowl. Add the unsalted butter pieces and work into the flour, creating thin sheets of butter in the flour.
Mix the egg yolk with 1 tablespoon water in a small bowl and add to the flour and butter mix. Continue to work with your fingers until the mixture sticks together when pinched. Add another tablespoon of water if needed.
Press the dough into a buttered tart pan with removable bottom (use either a long tart pan or a 10-inch round pan) or 4-6 individual tart pans with removable bottoms. Prick the bottom with a fork and then refrigerate for about an hour.
Preheat the oven to 375 degrees. Bake the tart for 12-15 minutes or until golden brown. Allow to cool completely before filling with lemon filling.
For the mascarpone filling:
Whip the cream on high with a hand mixer or a stand mixer. Add the softened mascarpone cheese and powdered sugar and beat mix well. Fold in the lemon curd with a wooden spoon.
In a separate bowl, gently mix blueberries and blackberries (well drained). Spoon the lemon cream into the cooled tart crust and top with berries.