How to Make It
In the top of a nonreactive double boiler set over but not touching simmering water, combine the eggs, egg yolks and sugar. Whisk frequently until the mixture is thick and pale yellow, about 10 minutes.
Whisk in the butter, piece by piece, allowing each piece to melt before adding the next. Add the grated zest and lemon juice and whisk frequently until the mixture is thick and the first bubbles appear, about 10 minutes longer; do not let boil.
Pour the filling into the Lemon Pastry Shell. Smooth the top with a spatula. Set the tart aside at room temperature until the filling firms up, about 30 minutes, before unmolding and serving. Garnish with the strips of lemon zest, if desired.