For the filling:
1 10-ounce package sweetened frozen raspberries, thawed, do not drain
1 1/2 cups whole frozen marion blackberries (or blackberries), thawed, do not drain
5 tablespoons sugar
3 tablespoons cornstarch
1 tablespoon lemon juice
1 tablespoon water
For the pastry:
1 1/2 cups all-purpose flour
1 cup finely ground hazelnuts
3/4 cup sugar
1 teaspoon cinnamon
1/4 teaspoon ground cloves
1/2 teaspoon finely shredded lemon peel
1/2 teaspoon finely shredded orange peel
1 teaspoon baking powder
6 tablespoons butter, softened
1 egg yolk
Cook's note: 16.5-ounce can of raspberries and a 16.5-ounce can marion blackberries may be used if frozen are not available. If canned berries are used, drain, reserving 3/4 cup juice and omit the additional 1 tablespoon water in recipe. Combine all filling ingredients in saucepan except berries, and heat until thickened. Add canned berries at this point and set aside to cool.
To make the filling: If using frozen berries, mix all filling ingredients in saucepan and simmer for 5-6 minutes, stirring gently until thickened. Set aside to cool. Prepare pastry.
To make the pastry: Preheat oven to 350 degrees. Mix all dry ingredients, lemon, and orange peels together. Using pastry blender or two knives, cut in butter very finely until mixture has the consistency of coarse sand. Beat egg and yolk together. Make a well in the center of the dry ingredients, pour in the beaten egg, and blend with a fork about 2-3 minutes, until all dry ingredients are moistened. It will be very soft. Note that the food processor method of cutting hard butter into flour is not recommended; it's essential to use softened butter to have a soft dough in order to pipe through a pastry tube.
To assemble the linzertorte: Press half the dough in the bottom of a 9-inch tart pan with removable bottom or 9-inch springform pan. Spread filling over dough. Using a pastry tube fitted with a 3/8–inch plain or ridged tip, pipe diagonal lattice over filling. Pipe a continuous border of large dots around the outer edge where lattice intersects. Bake at 350 degrees for 35 to 40 minutes. Cool tart completely before trying to unmold. Dust with powdered sugar.
Yield: 10 to 12 servings