Preheat the oven to 190 degrees Celsius (374 degrees Fahrenheit) (fan). Mix sugar, flour, salt, baking powder, and grated lemon zest together. Lightly beat the eggs with a fork and stir them into the flour mixture. Combine the dough and fold in the almonds and pistachios. Roll the dough into thumb-thick logs and place them on baking trays lined with parchment paper. There can be a maximum of two logs per tray as the dough spreads a lot. Bake the cookies in the oven for about 20 minutes until the dough is set. Take them out and let them cool slightly before brushing with beaten egg and water. Reduce the oven temperature to 150 degrees Celsius (302 degrees Fahrenheit) (fan). Cut the logs into slices about 2 cm thick. Spread them out on the baking trays and bake them crispy and hard in the oven for about 20 minutes. Take them out and let them cool completely.