To make the crust: In a medium-sized mixing bowl, whisk together the flour, sugar and salt., Using your fingers, a pastry fork or blender, mixer or food processor, work in the butter until crumbly., Add the grated lemon rind and juice, and press the mixture into the bottom of a lightly greased 9" x 13" pan., Bake the crust in a preheated 350°F oven for 10 to 15 minutes, until it's lightly browned. Remove it from the oven (leave the oven on) and allow it to cool slightly., While the crust is baking, make the filling. Beat the eggs and salt until foamy. Gradually add the sugar, beating until the mixture is thick and light yellow., Blend in the flour, then fold in the rhubarb., Pour the rhubarb mixture over the still-warm crust, return the pan to the oven, and bake the bars for 40 to 45 minutes, until they're lightly browned., Remove them from the oven, and cool for several hours for easiest cutting. The top layer separates a bit from the filling when you cut; just press it back lightly.