Line the bottom and sides of 8 or 9 x 3 springform pan with ladyfingers.
Soften the gelatin in lemon juice.
In top of double boiler off the heat and using a hand-held electric mixer or wire whisk, beat the yolks with 1/2 cup sugar, and salt until smooth.
Bring water in the bottom of a double boiler to a rapid simmer over medium-high heat. Set the top of the double boiler over the simmering water. Gradually whisk in the gelatin mixture and cook, whisking constantly, until the mixture starts to thicken, about 10 minutes.
Pour into a large bowl and fold in the lemon peel. Cover with plastic wrap and refrigerate until thickened, about 30 minutes, stirring occasionally (or chill over ice cubes in a larger bowl, stirring frequently, about 15 minutes).
In a heavy 2- to 3- quart saucepan, stir together the egg whites and remaining 1/2 cup of sugar. Over very low heat and using a hand-held electric mixer, beat egg whites until soft peaks form (or beat in bowl or double boiler over simmering water). Remove from the heat and cool slightly.
Fold the beaten egg whites and whipped cream into lemon-gelatin mixture. Spoon into the ladyfinger-lined pan. Chill for 3 hours or overnight.
Carefully remove the sides of the springform pan. Garnish with lemon cartwheel slices and fresh mint leaves, if desired.