Lemon Cream Tart with Gingersnap Crust

To make crust: Preheat oven to 350 degrees, placing a rack on middle rung. In a mixing bowl, thoroughly combine gingersnap crumbs and butter. Press evenly into bottom and sides of a 9-inch pie pan (crust should just reach rim). Bake 8 to 10 minutes, or until crust darkens at edges. Cool on wire rack. To make filling: In a small bowl, sprinkle gelatin over water. Separate egg yolks from whites and place in separate bowls (place yolks in a heatproof metal or glass bowl). Whisk egg yolks with 1/2 cup sugar until light and lemon-colored. Whisk in lemon juice. Set bowl over a saucepan of simmering water. Cook mixture, stirring constantly, 12 to 15 minutes, or until it thickens enough to coat the back of a spoon. Do not allow mixture to boil, or yolks will curdle. Remove bowl from heat and stir in softened gelatin until dissolved. Strain into a bowl, then stir in lemon zest. Refrigerate lemon mixture, stirring occasionally, until cold and thickened, about 20 minutes. In a large bowl, with an electric mixer on medium-high speed, beat egg whites until foamy. Turn mixer to high speed, then gradually beat in remaining 1/4 cup sugar, just until whites form stiff peaks. Transfer mixture to another large bowl and in the bowl you used to beat egg whites (no need to clean), beat cream until stiff. Fold lemon-egg mixture into beaten egg whites. Fold in whipped cream. Pour lemon mixture into cooled crust. Cover loosely and chill until set, about 6 hours. Arrange lemon slices around crust and place 1 slice in center of tart. Serve chilled.