In a medium metal bowl combine the yolks, egg, sugar, juice and zest and mix until well incorporated. Set the bowl over a double boiler and whisk constantly for 10-12 minutes. The mixture will turn into a soft yellow color and become thick. Once thickened, remove from the double boiler and stir in the butter a little at a time. Set aside.

In a mixer, whip the heavy cream until it becomes thick and fluffy

Combine the curd and the whipped cream and mix until evenly incorporated. Fill the tart shell and top with mixed berries.