Line the baking tray with parchment paper. Finely grind the walnuts and mix them with the sugar in a baking bowl. Lightly whisk the two egg whites and stir a good half of them into the nut-sugar mixture until the mixture is smooth and kneadable. If the dough is too dry, add a little more of the egg whites. Shape the mixture into two 30 cm long rolls. With a moistened finger, press long grooves about 1 cm deep into the rolls.
Place both pieces on the baking tray with enough space between them. Brush the edges with the remaining egg white and sprinkle with pistachios. Bake in a heated kitchen oven (electric oven: 180 °C gas: level 2 to 3, convection oven: 160 °C) for about 15 to 20 minutes until they are delicately browned.
Heat the jam in a saucepan and stir until smooth. Fill the grooves with the jam after baking. After cooling, cut into 24 pieces.