• To make the lemon curd, beat together the egg yolks and sugar, in a medium saucepan.
• Pour in the lemon juice and zest.
• Stir continually and place the saucepan over medium-low heat.
• Continue stirring until the mixture is thick enough to coat the back of a spoon.
• Strain mixture into a shallow pan and discard the zest.
• Allow the mixture to cool slightly.
• Then, add butter if using, and whisk.
• Next, place plastic wrap over the lemon curd, directly on the surface.
• Cool to room temperature.

To Make the Meringue

• Then to make the meringue, bring a medium pot of water to a simmer.
• Place the egg whites and sugar in a medium bowl and whisk.
• Set eggs over simmering water.
• Whisk until the mixture thickens and reaches 160 F on a thermometer.
• Then, place the egg whites in the bowl of an electric mixer, fitted with a whisk attachment.
• Add cream of tartar.
• Whip on high speed until soft peaks form.

To Assemble:

• To assemble, smooth the lemon curd into the tart evenly.
• Swirl meringue decoratively over the tart, making sure to spread right to the edges.
• Since this meringue has already been cooked, set it under the broiler for 2 minutes just to get a little colour.
• Serve immediately.