Sprinkle gelatin over cold water in a small bowl, and let stand until softened, about 5 minutes.
Whisk together egg yolks, sugar, and salt in a large heatproof bowl. Gradually whisk in lemon juice. Place bowl over a saucepan of simmering water, and whisk constantly until mixture has thickened and registers 160 degrees on an instant-read thermometer. Whisk in gelatin mixture. Remove from heat, and whisk in butter, a few pieces at a time, until smooth. Let cool, stirring occasionally. Prepare an ice-water bath. Place bowl of yolk mixture over bath, and stir until slightly thickened, about 2 minutes.
Spread curd into crust; smooth top. Dollop creme fraiche on top. Using a wooden skewer or the tip of a knife, swirl creme fraiche into curd to create a marbleized effect. Refrigerate until firm, at least 2 hours (or up to overnight).