Preheat the oven to 350°F. Prepare an 8x11 (or 9x13) pan by lining it with parchment paper. It helps to butter the pan a little first so that the parchment paper sticks.
Mix the flour, sugar, and salt in a large bowl with a whisk. Add the lemon zest and press it through the mixture with your fingers.
Using a grater, grate the chilled butter into the flour, sugar, salt, and zest mixture stopping occasionally to toss it with the flour mixture.
Press into the prepared pan and bake until golden, about 20 minutes.
FOR THE LEMON CUSTARD
Place the flour and sugar in a bowl and add the lemon zest. Use your fingers to press it through the mixture.
Add the eggs and lemon juice and whisk till it is incorporated. Pour the custard over the crust and return it to the oven till it is just set. 20-30 minutes. If you use the smaller 8x11 pan, you'll probably have to go longer than 30 minutes. Test it by slightly jiggling the pan. It should have a little jiggle, but not much.
Allow to cool at room temperature before placing the pan in the fridge. Refrigerate for at least 2 hours before serving. Store in the refrigerator.