For the crust—In a food processor, grind the walnuts to a fine grind. You should have about 1/2 cup. Using an electric mixer with the paddle attachment, beat the butter and confectioners’ sugar at high speed until light and fluffy. Add the egg; mix to combine. Lower the speed to slow and add the flour, mixing until barely combined. Add the walnuts and continue mixing, scraping the sides of the bowl, until the dough comes together. Divide the dough. Wrap the half you’ll be using in plastic and refrigerate for at least 2 hours or overnight. Wrap the other half well in plastic and then in foil and freeze for future use.
Heat the oven to 400°F. Lightly oil a 9-1/2-inch tart pan. On a lightly floured surface, roll the dough into a round about 1/8 inch thick. Arrange it in the pan, trimming to fit. Line the crust with foil or kitchen parchment and weight with beans or pie weights. Bake until the edge is light golden brown, about 20 minutes. Carefully remove the beans and foil and bake until the bottom is dry and light brown, about another 5 minutes. Cool to room temperature.
For the filling—Heat the oven to 350°F. In a medium bowl, whisk the eggs. Add the sugar and whisk until just combined. Add the lemon juice and cream and whisk until just combined. Strain the mixture through a fine strainer and pour into the prepared pie crust.
Reduce the oven temperature to 325°F. Bake the tart until the filling is set, 25 to 30 minutes. Cool at room temperature. Serve at room temperature or chilled.