Preheat oven to 180 degrees Celsius (350 degrees Fahrenheit).

In a large bowl, combine plain flour, sugar, almond flour, egg, baking powder (if using, inferred), lemon zest, and water.

Mix all ingredients thoroughly until a dough forms.

Add the unskinned almonds and hazelnuts to the dough.

Divide the dough into two equal parts.

Shape each part into a cylinder approximately 3 cm (1 inch) in diameter.

Place the dough cylinders on a baking sheet lined with parchment paper.

Bake in the preheated oven for about 25 minutes, or until lightly golden.

Remove from the oven and let the cylinders cool slightly.

Using a serrated knife, cut the cylinders on the bias into 1 cm (1/2 inch) thick slices.

Arrange the biscotti slices on the baking sheet.

Return the biscotti to the oven and bake for another 10 to 15 minutes, or until they are golden brown and crunchy.

Remove from the oven and let the biscotti cool completely on a wire rack.

Store the biscotti in an airtight container, such as a glass or metal box, to maintain their crispness.