Preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper and set it aside.
In a small bowl, combine the granulated sugar and fresh lemon zest. Use your fingers to rub the zest into the sugar until it becomes fragrant and looks like wet sand. This crucial step releases the essential citrus oils for maximum aroma.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, salt, and the lemon-infused sugar.
In a separate bowl, whisk the eggs, fresh lemon juice, vanilla extract, and almond extract until fully combined.
Pour the wet ingredients into the dry flour mixture. Stir with a sturdy spatula until a stiff dough starts to form. Gently fold in the sliced almonds until they are evenly distributed.
Turn the dough out onto a lightly floured work surface. Divide it in half and shape each portion into a log about 9 inches long and 2.5 inches wide. Place them on the prepared baking sheet with a few inches of space between them.
Bake for 25 minutes, or until the logs are firm and lightly golden on top.
Remove from the oven and allow the logs to cool on the baking sheet for exactly 10 to 15 minutes. Lower the oven temperature to 325°F (165°C).
Transfer the slightly cooled logs to a cutting board. Using a sharp serrated knife, slice them on a slight diagonal into 3/4-inch thick pieces.
Arrange the biscotti slices cut-side down on the baking sheet. Bake for 8-10 minutes, flip them over, and bake for an additional 8-10 minutes until completely dry and slightly golden. Let them cool fully on a wire rack before serving.