In a medium saucepan over low heat, whisk together the lemon juice, eggs and sugar until well blended. Cook, stirring constantly with a heat-proof spatula, 5 minutes or until the mixture is hot. Whisk in the butter, a few pieces at a time, until just melted and fully incorporated. Continue to cook, stirring constantly and scraping bottom of pan with spatula, 3 to 5 minutes until the filling is thick and smooth. Do NOT let mixture boil, or it may curdle. Scrape the filling through a fine-mesh sieve into a medium bowl. Stir in the lemon zest.