Prepare an ice-water bath; set aside. Place 1 tablespoon water in a small saucepan. Sprinkle gelatin over and let soften, about 5 minutes. Add lemon juice and sugar. Place over low heat and cook, stirring, until gelatin has dissolved. Remove from heat and stir in lemon zest. Place over prepared ice-water bath until chilled and syrupy.
Place egg whites in the bowl of an electric mixture fitted with the whisk attachment and whisk on medium-high speed until stiff peaks form. With machine running, add lemon-gelatin mixture and whisk just until incorporated.
In a separate bowl of an electric mixture with a clean whisk attachment, whisk heavy cream until thick; fold into lemon-egg white mixture.
Transfer mixture to a large bowl or individual serving dishes and chill for 3 hours before serving.