Preheat the oven to 350 F. To prepare crumb crust/topping, spread nuts in a medium baking pan. Roast the nuts, stirring occasionally, until toasted, about 10 minutes. Remove nuts from the oven and turn out onto a cloth towel. Rub off and discard the papery skins. Increase the oven temperature to 450 F. In a food processor fitted with the metal blade, process the nuts until finely ground, about 10 seconds. In a large bowl, mix together the nuts, flour, and sugar. Using a pastry blender or 2 knives, cut the butter into the flour mixture until coarse crumbs form. Using fingers, evenly press half of the crumb crust/topping mixture into the bottom and sides of an 8- or 9-inch tart pan.
To prepare the filling, in a medium bowl, mix together the sugar and the cornstarch. Gently fold in the berries. Spoon the berry mixture into the crust, spreading evenly. Sprinkle the berry mixture evenly with the remaining crumb crust/topping. Bake until topping is golden and filling is bubbly, 30 minutes. Transfer the pan to a wire rack to cool for 10 minutes. Serve warm.