Preheat oven to 350ºF (177ºC). Line a large baking sheet with parchment paper or a silicone baking mat. Lightly flour the baking sheet. Set aside.
Place the sugar in a medium size bowl and add the lemon zest. Using your fingers, rub the zest into the sugar until all of the zest has been incorporated and most if not all of the sugar is pale yellow. Whisk in the butter, oil, egg, vanilla extract, and 4 teaspoons of the lemon juice until combined. Set aside.
In a large bowl, toss together the flour, baking powder, salt, and poppy seeds. Add the wet ingredients to the dry ingredients and gently mix with a large wooden spoon or sturdy rubber spatula until everything is combined.
Check out this Instagram reel to see how I make my biscotti. Turn dough out onto lightly floured baking sheet and, with floured hands, pat dough into a rectangle about ½" thick. My rectangle was 6" wide and 7 and ½" long (using a ruler or a ruled silicone baking mat is helpful here). Make the egg wash using the remaining lemon juice (approximately 5-6 teaspoons) and brush onto the entire rectangle, being sure to get the egg wash on the sides of the dough as well.
Bake dough slab for 22-24 minutes, or until the top and sides of the slab are lightly browned. Remove from the oven, but do not turn off the heat. Place baking sheet on a wire rack and allow to cool for 10 minutes.
Once the dough is cool enough to touch, cut into 1" thick slices using a sharp knife. Cut these 1" slices in half, making a perpendicular slice down the entire slab. Set slices cut sides up on the baking sheet.
Return to the oven to bake for 8 minutes. Remove from oven, turn biscotti over, and bake the other side for 8 minutes. Remove from the oven and allow to cool for 3 minutes on the baking sheet (cookies will become crunchy as they cool). Transfer biscotti to a wire rack to cool completely. Store biscotti in an airtight container for up to 2 weeks. Baked biscotti may be frozen up to 3 months.