PreparationFor crust: Blend flour, almonds, walnuts, and salt in processor until nuts are finely ground. Add powdered sugar and combine using on/off turns. Add butter; using on/off turns, cut in until coarse meal forms. With machine running, slowly add egg. Blend until moist clumps form. Gather dough into ball; flatten into disk. Wrap in plastic and chill 30 minutes. Roll out dough on lightly floured surface to 14-inch round. Transfer dough to 10-inch-diameter tart pan with removable bottom. Trim excess dough flush with top edge of pan. Pierce bottom of dough all over with fork. Chill 30 minutes. Preheat oven to 325°F. Bake crust until golden, about 35 minutes. Transfer to rack and cool completely. (Can be made 1 day ahead. Cover and let stand at room temperature.) For filling: Place sugar, 1/2 cup water, and corn syrup in heavy large saucepan. Stir over low heat until sugar dissolves. Increase heat and boil without stirring until syrup is deep amber color, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 8 minutes. Remove from heat. Whisk in butter and cream (mixture will bubble vigorously). When bubbling subsides, whisk in crème fraîche. Cool caramel until beginning to thicken. Stir in walnuts. Pour caramel into prepared crust. Let stand 15 minutes to spread. For chocolate glaze: Bring cream to boil in medium saucepan. Remove from heat. Add chocolate and stir until melted and smooth. Cool 5 minutes. Pour enough glaze over caramel to cover and fill tart to just below edge of crust (reserve remaining glaze for another use). Let stand until set, about 2 hours. (Can be made 1 day ahead. Cover and refrigerate. Bring to room temperature before serving.)
PreparationFor crust: Blend flour, almonds, walnuts, and salt in processor until nuts are finely ground. Add powdered sugar and combine using on/off turns. Add butter; using on/off turns, cut in until coarse meal forms. With machine running, slowly add egg. Blend until moist clumps form. Gather dough into ball; flatten into disk. Wrap in plastic and chill 30 minutes. Roll out dough on lightly floured surface to 14-inch round. Transfer dough to 10-inch-diameter tart pan with removable bottom. Trim excess dough flush with top edge of pan. Pierce bottom of dough all over with fork. Chill 30 minutes. Preheat oven to 325°F. Bake crust until golden, about 35 minutes. Transfer to rack and cool completely. (Can be made 1 day ahead. Cover and let stand at room temperature.) For filling: Place sugar, 1/2 cup water, and corn syrup in heavy large saucepan. Stir over low heat until sugar dissolves. Increase heat and boil without stirring until syrup is deep amber color, occasionally brushing down sides of pan with wet pastry brush and swirling pan, about 8 minutes. Remove from heat. Whisk in butter and cream (mixture will bubble vigorously). When bubbling subsides, whisk in crème fraîche. Cool caramel until beginning to thicken. Stir in walnuts. Pour caramel into prepared crust. Let stand 15 minutes to spread. For chocolate glaze: Bring cream to boil in medium saucepan. Remove from heat. Add chocolate and stir until melted and smooth. Cool 5 minutes. Pour enough glaze over caramel to cover and fill tart to just below edge of crust (reserve remaining glaze for another use). Let stand until set, about 2 hours. (Can be made 1 day ahead. Cover and refrigerate. Bring to room temperature before serving.)