Preheat the oven to 250 degrees F. Line 2 cookie sheets with parchment paper or non-stick oven liner film.
Put the egg whites and lemon juice in a chilled bowl. Beat with an electric mixer to a soft foam. Slowly beat in the sugar, mixing well after each addition. Continue beating at medium-high speed until stiff peaks form. Add the vanilla and salt. Gently fold in the nuts.
Drop the batter by heaping teaspoons onto the cookie sheets.Shape with the back of a spoon onto 1-inch circles. Bake until set and firm, about 1 hour. Turn off the oven but leave the cookies there until the oven is completely cool. Remove the cookies from the oven and store in an airtight container at room temperature.