Spread pecans on a 10- by 15-inch baking sheet. Bake in a 325° oven until very slightly darker, about 10 minutes. Let cool; leave oven on.
Meanwhile, in a large bowl, with a mixer on medium speed, beat butter and sugar until well blended. Add eggs, vanilla, lemon peel, and lemon juice and beat until combined.
Add flour, polenta, baking powder, and salt and beat until combined (dough will be sticky). Stir in pecans.
Divide dough in half and place portions several inches apart on a baking sheet lined with cooking parchment. Pat each into a log about 10 inches long, 2 inches wide, and 1 inch tall (moisten your hands with cold water if necessary to prevent sticking).
Bake until logs start to turn golden at the edges, about 25 minutes. With a wide spatula, transfer to a wire rack and cool for 10 to 15 minutes.
With a heavy, sharp knife, cut each log crosswise into 3/4-inch-thick slices. Place slices on their side on the cooking parchment-lined sheet. Return biscotti to oven and bake until golden on the bottom, about 20 minutes. Turn them over and continue baking until cookies are crisp and golden on the other side, about 20 minutes longer. Transfer biscotti to a rack to cool. Store airtight up to 2 weeks.
Nutritional analysis per cookie.