Preheat oven to 350°F (177°C). Line a 9×9 pan with foil, leaving a 2-inch overhang. Lightly spray with nonstick spray.

Mix shortbread crumbs and melted butter until sandy. Press into pan and bake for 14 minutes, until edges are lightly golden. Cool slightly.

Whisk sugar, salt, and zest. Add lemon juice and eggs one at a time, mixing until smooth.

Pour filling over warm crust. Bake 18–22 minutes until edges are set and center slightly jiggles. Cool 30 minutes, then chill 3 hours.

Over simmering water, whisk egg whites, sugar, and tartar until 135°F or sugar dissolves. Beat until stiff, glossy peaks form.

Lift chilled bars from pan. Spread meringue in swirls. Torch or broil 20–30 seconds until golden.

Let sit 5 minutes. Slice into 12 bars with a hot knife. Serve fresh.