Preheat oven to 350F. Line 9×9-inch baking pan with aluminum foil.
Make the crust by whisking together flour, sugar, salt and lemon zest. Cut butter into chunks and add to flour mixture. Blend in with an electric mixer at low speed until mixture forms coarse, sandy crumbs. Pour into pan and press mixture down (with your fingers or the back of a spoon) into an even layer.
Bake the crust for 16-19 minutes, until just lightly browned around the edges.
While crust is baking make the filling by whisking together all filling ingredients in a large bowl.
When the crust comes out of the oven and is still hot, pour in filling mixture.
Bake for about 20 minutes, or until filling is set and does not jiggle when the pan is gently shaken.
Cool completely before slicing.