Whisk 2 egg whites, 1/4 cup sugar and a pinch of salt in a heatproof bowl set over a pan of simmering water until the sugar dissolves. Remove the bowl; beat with a mixer until stiff peaks form. Whisk 1 1/2 cups low-fat Greek yogurt, 1 teaspoon lemon zest and 1 tablespoon lemon juice in a separate bowl; fold in the egg whites. Chill 2 hours. Serve with crumbled shortbread cookies.