In a mixing bowl gently mix together the blackberries, blueberries, raspberries, sugar, and until evenly coated.
In a food processor combine the flour, salt, brown sugar, oats, and walnuts. Pulse until evenly mixed. Add the melted butter and pulse until combined.
Roll out the pastry dough thinly and place in the pastry shell. Slice off any excess dough.
Add the cornstarch to the filling, pour out any liquid, and fill the tart shell with the berry mixture. and then top with the crumble. Bake for 40 minutes or until the shell and crumble have turned a golden brown. Allow to cool before serving and let the filling set.