Reduce oven to slow (150 degrees celsius, fan-forced).
Beat egg yolks until they thicken a little and turn a more pale colour.
Gradually add condensed milk, 1/2 cup lemon juice and zest from 3 limes, beating after each addition. Then beat again for one minute.
Beat egg whites in a separate bowl until soft peaks form.
Fold egg whites through the filling mixture until combined and then pour into the cooled tart shell.
If you're making the optional meringue topping, beat the egg white until soft peaks form and then gradually add sugar. Beat until glossy and thick. Dob or pipe meringue mixture around the edge of the pie.
Bake for 15-18 minutes. The filling should be set but still a little wobbly when you remove it from the oven.
Allow to cool and then chill in the fridge for at least 3 hours.
Cut a few slices of the extra lime and place on top of tart. Sprinkle with zest from the extra lime.
Serve chilled with cream or ice cream.