1-Preheat oven to 350 degrees Fahrenheit: Preheat oven to 350 degrees Fahrenheit. Very lightly grease or line an 11×7-inch baking dish with parchment paper that overhangs 2-inches on both sides.
2-To make the crust: To make the crust: Combine the flour, powdered sugar, salt and butter in a food processor. Pulse until mixture resembles fine crumbs. Spread evenly, then press into prepared pan and bake for 18 to 20 minutes, or until golden brown.
3-Meanwhile make the filling: Meanwhile make the filling: Whisk together the sugar and flour in a bowl. Add the eggs, one at a time, whisking well after each addition. Add the zest, lime juice, and a drop of green (+ yellow) food coloring, if desired. Mix until everything is smooth and combined.
4-Pour filling over hot crust and return: Pour filling over hot crust and return to oven to bake for an additional 18 to 20 minutes or until set. When done, the filling will no longer jiggle.
5-Let cool completely in the pan, about: Let cool completely in the pan, about 1 hour. Then refrigerate for 1-2 hours and serve cold. Lift bars out of pan with parchment paper for easy cutting and serving. Dust with powdered sugar, if desired. Store leftovers in the refrigerator.