Make the crust: Whisk together flours, sugar, and salt in a bowl. Using a pastry blender or two forks, cut butter into flour mixture until it’s the consistency of crumbs. Add egg and mix to combine. Using your hands, work dough, gently squeezing, until a ball forms, about 1 minute. Divide dough in half and flatten each into a disc. Wrap in plastic wrap. Chill for at least 30 minutes and up to 2 days.

Preheat oven to 375°F. On a lightly floured work surface, roll out one portion of dough into an 11-inch circle. Transfer to a 9-inch tart pan with a removable bottom, pressing dough into the base and side flutes; trim excess. Freeze for 20 minutes. Line tart shell with aluminum foil or parchment paper, and fill with pie weights, rice, or dried beans, pressing all the way to the edges.

Bake 15 minutes. Carefully remove foil and pie weights. Bake until edges are light golden brown and bottom is dry, 8 to 10 minutes. Transfer to a wire rack and cool completely. Once cool place on a rimmed baking sheet.

Make the filling: Reduce oven to 350°F. Whisk together eggs in a bowl just until combined. Whisk in sugar, lemon juice, and heavy cream just until combined. Transfer to tart shell.

Bake until set but still wobbly in the center, 20 to 25 minutes. Transfer to a wire rack, and gently push on removable bottom to release the crust from the sides (do not remove it from ring). Once cool, remove ring. Serve at room temperature.