Preheat your oven to 350°F.

In a medium saucepan, combine the lemon juice and sugar, stirring until dissolved. Gently bring to a simmer without boiling.

In a separate bowl, whisk together the eggs until well blended and slightly frothy.

Remove the lemon mixture from heat and slowly stir in the whisked eggs, mixing continuously to prevent scrambling.

Return to low heat and add the unsalted butter, stirring until melted completely.

Stir in the vanilla extract.

Pour the lemon filling into the premade tart shell, being careful not to overflow.

Bake at 350°F for about 20 minutes until the filling is set but still slightly jiggly in the center.

While the tart cools, prepare the meringue. In a clean bowl, combine egg whites and cream of tartar. Beat until soft peaks form.

Gradually add the 1/2 cup of sugar while continuing to beat until glossy with stiff peaks.

Spread meringue over the cooled lemon filling, ensuring it touches the edges.

Bake again at 350°F for 10-15 minutes until meringue is lightly browned.

Allow the tart to cool completely before cutting.

Slice into small pieces with a sharp knife and serve either chilled or at room temperature.