Set aside ¼ cup of the sugar and put ¾ cup in a mixing bowl. Add the lemon zest and mash it around with the back of a spoon until you have a fragrant, lemony sugar. Add the the salt and the flour and stir until well combined. Add melted butter, lemon juice, and egg yolks. Mix until well blended. Stir in cream and milk. Set aside.

In a mixing bowl, beat egg whites and cream of tartar until fluffy. Gradually add the reserved ¼ cup sugar and beat until stiff but still moist.

Fold egg white mixture into lemon mixture. Pour batter into the baking dish. Place the baking dish in the larger pan and fill the larger pan with water so that it comes about halfway up the sides (water bath). Bake on center rack for 40-45 minutes or until top is lightly browned. Serve hot or cold. Serves 6.