Preheat oven to 325 degrees. Put egg white and salt in the bowl of an electric mixer fitted with the whisk attachment, and mix on medium-high speed until stiff peaks form. Reduce speed to medium; mix in nuts and sugar, then flour and lemon juice.
Using a 1 1/4-inch ice cream scoop, drop dough onto baking sheets lined with parchment paper, spacing 2 inches apart. Bake cookies, rotating sheets halfway through, until edges are golden, 10 to 12 minutes. Let cool on sheets on wire racks. Cookies can be stored between layers of parchment in airtight containers at room temperature up to 3 days.