Lightly grease 2 cookie sheets or line with parchment paper; set aside. In bowl, mix flour, baking powder, salt, and baking soda.
In a large mixing bowl, beat butter with an electric mixer for 30 seconds. Add granulated sugar and lemon peel. Beat until combined, scraping sides of bowl. Beat in the 2 eggs, one at a time, until combined. Beat in lemon juice until combined. Beat in as much of the flour mixture as you can with the mixer. Using a wood spoon, stir in remaining flour mixture. Stir in walnuts.
Turn dough out onto a lightly floured surface; divide into three equal parts. Shape each portion into a 10-inch-long loaf about 1-1/2 inches wide. Place loaves about 3 inches apart on prepared cookie sheets; flatten loaves slightly until about 2 inches wide. Brush loaves with egg white. Sprinkle with turbinado sugar. Place cookie sheets in refrigerator and chill for 15 minutes.
Bake in a 325 degree F oven for 25 to 30 minutes, or until firm and light brown. Cool on cookie sheets for 45 minutes. Reduce oven temperature to 300 degrees F.
Transfer loaves to a cutting board. Use a serrated knife to cut each loaf diagonally into 1/2-inch slices. Put slices on cookie sheets, cut sides down. Bake about 7 minutes, or until lightly browned. Turn the cookie slices over; bake for 5 to 6 minutes more or until lightly browned, crisp and dry. Remove and cool on wire racks Store in an airtight container at room temperature, or freeze the biscotti for up to 3 months. Makes 50 cookies.