In a large bowl, combine the dry ingredients. In a small bowl, combine the oil, eggs, and lemon zest. Add egg mixture to the dry mixture and mix until just combined. DO NOT OVERMIX.
Shape into a flat rectangle on a parchment-lined or silpat-lined baking sheet. The rectangle should be about 5x7 inches.
Bake for 25 minutes. Let cool slightly and then cut into 12 slices. (I cut off the ends and snack on those right away!) Lay the slices on their sides on the lined baking sheet.
Reduce oven to 300F and bake slices for 10 minutes. Flip the biscotti and bake another 10 to 12 minutes until crisp but not browned. Remove to a cooling rack and leave to cool completely.
In a bowl, whisk together the powdered sugar and the lemon juice until smooth and thin enough to drizzle. If it is too thick, add a tiny bit more lemon juice. If it is too thin, add a little powdered sugar. Drizzle over the cooled biscotti and allow to set until dry.