In a food processor fitted with the plastic blade (or by hand with a pastry blender in a large mixing bowl), mix all crust ingredients until the mixture resembles coarse meal. Put the mixture in the prepared pan, roll it (I use a rolling pin that fits exactly into the pan) or press it with your fingers until you have a smooth 1/4 inch layer on the entire bottom and about 1/2 inch up the sides. Chill the pan in the freezer for 15 minutes and then bake for 22 minutes in the preheated oven.
While the crust is baking, prepare the filling: Put the steel blade in the food processor and add the granulated sugar and lemon peel (if you don't have a food processor, you can grate the lemon peel and mix the filling by hand with a whisk); pulse until the sugar is yellow and all pieces of the lemon peel have been finely chopped. Add the remaining filling ingredients and pulse until smooth.
When the crust has finished the first 22 minutes of baking, lower the oven temperature to 325°F., remove the crust, pour the filling over the crust, and return the pan to the oven to bake for 20-25 minutes. You will know that the bars are done when the filling is just firm to a light touch.
Cool the pan on a rack until room temperature. Remove the entire pastry to a cutting board by lifting up on the parchment paper. Sprinkle with confectioner's sugar, if desired. Then cut into bars with a sharp knife or a pizza cutter. To freeze, cut first, then wrap very tightly in several layers of foil.
Makes about 24 2" squares.