Place the roasted hazelnuts in a food processor and pulse until finely ground, but not yet turning into a paste. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment or a large bowl, beat together the butter and powdered sugar until light and creamy.
In another bowl, combine the ground hazelnuts, cake flour, vanilla sugar, lemon zest, baking powder, cinnamon, cloves, and salt.
Beat the egg into the butter, then the hazelnut mixture just until a smooth dough comes together.
Form the dough into a disc, wrap in plastic, and refrigerate for one hour.
Preheat the oven to 350˚F (180˚C). Grease a round 9 inch (23 centimeter) springform pan along the bottom and sides with butter. Cut out a 9 inch (23 centimeter) circle of parchment and place along the bottom of the pan.
Remove the chilled dough from the refrigerator and place on a floured surface.
Cut off 1/4 of the dough and set aside.
Roll the remaining 3/4 of the dough into a circle slightly larger than 9 inches (23 centimeters) and gently transfer to the prepared pan. Press the dough across the bottom until even and about 1 inch (2.5 centimeters) up around the sides. If uneven, carefully use a butter knife to cut away the excess.
Evenly spread the red currant jelly across the base of the dough, taking care to not get it on top of the sides.
Roll the remaining dough into 6 ropes about 1/2 inch (1.25 centimeters) thick.
Arrange the ropes gently over the jelly in a grid-like/lattice pattern- forming either diamonds or squares, 3 in each direction. Use a butter knife to cut away the excess.
If desired, roll the excess dough into another rope to wrap around the perimeter of the torte and seal in the top strips.
In a small bowl, beat the egg until smooth.
Lightly brush the egg around the edge and across the strips of dough, taking care to not let any fall into the jelly.
Sprinkle the sliced almonds around the edges of the torte.
Place the springform pan on a rimmed baking sheet and bake the Linzer Torte in the preheated oven until golden brown and set, 35-45 minutes.
Allow the torte to cool completely to room temperature before removing from the springform pan.
For best results, store the Linzer Torte in an airtight container at room temperature for at least a day and ideally 3 days before slicing and serving.