Gather the ingredients for the filling.
Set up a double boiler over medium-low heat, or use a large heatproof bowl over a saucepan with a few inches of simmering water, ensuring the bowl does not touch the water. Add the butter and crème fraîche, stirring until the butter melts and the mixture is smooth and warm. Remove from heat and set aside.
In another heatproof bowl over the double boiler, whisk together the eggs, egg yolks, and sugar until the mixture becomes slightly pale. Slowly incorporate the lemon juice while continuously stirring. Cook for 3 to 4 minutes, stirring constantly, until the mixture thickens.
Add the reserved butter-crème fraîche mixture and the lemon zest, stirring until fully blended. Continue cooking for another 3 minutes until the filling is warmed through and velvety.
Reduce the oven temperature to 300°F.
Carefully pour the filling into the prepared tart shell, smoothing it out evenly. Bake for 15 to 20 minutes, or until the filling is set and takes on a golden hue.
Allow the tart to cool in the pan on a wire rack for 15 minutes before serving. It can be enjoyed at room temperature or chilled for a firmer texture. Garnish with lemon zest, fresh citrus slices, or a dollop of crème fraîche or whipped cream, if desired.