In the top of a double boiler over simmering water, cook the sugar, lemon juice, butter, zest, and eggs, whisking frequently, until a custard forms and bubbles appear on surface, 10 to 15 minutes. Remove from the heat and strain through a fine mesh sieve into a bowl.

Spoon the lemon curd into the prepared tart shell, smoothing the top with the back of a spoon or rubber spatula. Arrange the berries in a decorative pattern over the top, and serve with whipped cream on the side, if desired.