In a large nonreactive saucepan, combine eggs, egg yolks, lemon zest, lemon juice, and sugar over low heat; whisk until incorporated. Add half the butter, and continue to whisk to prevent mixture from curdling. Once mixture thickens enough to coat the back of a spoon, add remaining butter and stir until very thick. (If mixture curdles, quickly remove from heat; strain into a bowl, and immediately cool over an ice bath.) Remove from heat, and continue stirring until lukewarm. (Speed up the cooling process by transferring the filling to a bowl resting in an ice bath.)
Heat broiler, if using. Spoon lemon filling into tart shell. Prevent crust from burning by placing a tart ring upside down over top edges of tart. Place under broiler for 5 to 10 minutes, or until top is speckled with brown, caramelized spots; rotate tart, if necessary. Alternatively, you may also use a small kitchen torch. Serve with a dollop of creme fraiche.