In the bowl of a food processor add the flour, ground almonds, sugar, and kosher salt. Pulse a few times to mix the dry ingredients. Cut the cold butter into small pieces and add to the food processor. Pulse until the mixture resembles small pea-sized pieces.
Pour the ice water and almond extract through the chute and pulse just until the dough begins to clump together. Dump the contents onto the counter and form into a disc. Wrap with cling film and let chill for 30 to 60 minutes.
Filling and Assembly
Preheat the oven to 400° F. Line a rimmed baking sheet with parchment paper and set aside. Wash the blackberries.
On a lightly floured surface roll the tart dough out into a 12-inch circle. Be gentle while rolling the dough to avoid making large rips – because this galette is free-formed any cracks will cause the filling to leak.
Gently transfer the dough to parchment-lined baking sheet. Carefully mend any cracks in the dough as best you can by pinching the dough together. Toss the blackberries with the flour, sugar, pinch kosher salt, crème de cassis and lemon juice. Spoon the berry filling into the middle of the dough. Spread the berries out evenly while leaving a 1 ½ inch edge border.
To form the tart, begin folding the crust up over the edge of the berries, overlapping and pleating the dough as you go. If the dough tears, then pinch it back together as best you can. Brush the edge of the crust with the beaten egg and sprinkle with turbinado sugar.
Bake the tart for 40 to 50 minutes until the pastry is golden browned and the blackberry filling is bubbling and slightly thickened. Let the tart cool for at 20 to 30 minutes before slicing.
Honey Whipped Mascarpone
In the bowl of a mixer fitted with a whip attachment add the whipping cream. Begin to whip the cream on high until it begins to thicken into soft peaks. Add the honey and mascarpone. Continue whipping until the cream has thickened into stiff peaks. Dollop the cream on slices of the blackberry almond tart!