Heat the oven to 150 C / 300 F (140 C / 275 F for a fan oven) and line two 33 x 46 cm / 13 x 18 inch sheet pans with parchment. In a 26 cm / 10-inch mixing bowl, whisk the egg whites, sugar, and salt for about 1 minute until foamy and the sugar looks wet.
Stir in the almond flour, 150 g flour, lemon zest, and almond extract with a wooden spoon, then knead in the bowl for 1 minute until a stiff dough forms. If it's crumbly, knead in water 1 tsp at a time. Cover and rest the dough for 30 minutes.
Divide the dough into 25 g pieces, about 26 in all. On a lightly floured surface, roll each piece into an 8 cm log about 1.5 cm thick. Press a shallow notch into each end with the back of a knife and round the ends slightly so they look like bone knobs.
Place 13 cookies on each pan, 4 cm apart, since they barely spread. Bake for 35 minutes, swapping and rotating the pans at 18 minutes, until the cookies are pale gold with a dry, matte surface and a lightly golden base.
Cool the cookies on the pans for 20 minutes, then move them to a wire rack. They firm up and turn hard as they cool.