Preheat oven to 350 degrees F. Set the tarts on a parchment paper-lined baking sheet. Place the filling ingredients in a bowl and whisk to combine. Pour filling into the tart shells. Bake 25 minutes, or until filling is set. Cool tarts to room temperature.
Preheat the oven to 400 degrees F. To make meringue, beat the egg whites and cream of tartar until soft peaks form. Gradually beat in the 1/2 cup sugar until stiff peaks form. Divide and mound meringue on top of each tart, ensuring the lemon filling is completely covered.
Bake the tarts 8 minutes, or until the meringue has nicely browned. Let tarts cool to room temperature and set before enjoying.