Preheat oven to 100 °C (210 °F).

Mix the egg whites until soft peaks form. Then add the sugar in batches while mixing all the time, add about 1 tablespoon of lemon juice, and mix until meringue becomes glossy. Add lemon juice gradually, then taste, the meringue should not be too sour.

Divide meringue in half, and in one half add ground walnuts and gently stir with a spatula, until walnuts are fully incorporated.

Flour well the working surface and then roll out the dough until it becomes about 1 cm thick. The dough is pretty sticky, so you might feel that you've done something wrong, but you haven't (if you feel the dough way too sticky, add a bit more ground walnuts, you can not do anything wrong).

Cut equal little bars out of the dough (this is "sticky" business, so after each cut, wash your knife or wipe it with a paper towel) and transfer them onto the baking sheet lined with parchment paper. * see note

Put the other half of meringue into a piping bag and squeeze onto each bar.

Put the bars in the preheated oven, and dry them about 1 - 1 1/2 hours, until meringue is firm but still white (do not let meringue turn yellow).

Let cookies cool to room temperature, then store them in an airtight container up to 5-7 days.