In a small separate bowl mix salt in lemon juice to dissolve then add to butter mixture along with the zest, and mix.
Add the flour and mix well, then fold in the walnuts as best you can.
Knead the dough lightly, enough to evenly distribute the walnuts, then split in half and roll into two logs about 2 inches thick. Wrap each log in plastic wrap and refrigerate for 2 ½ to 3 hours to firm up.
Preheat oven to 350 degrees and line baking sheets with parchment paper.
Cut each cookie dough log into slices about ⅜" thick. Arrange on parchment. These shortbread cookies will not expand much at all, but leave a little space between them.
Bake for 11-12 minutes, then remove to rack to cool.