Preheat your oven to 180 °C (350 °F).
In a clean, dry mixing bowl, add the egg whites and cream of tartar. Using an electric mixer or a stand mixer fitted with the whisk attachment, beat the egg whites on medium-high speed until they form soft peaks. Soft peaks are reached when the whites hold their shape, but the tips curl over when the beaters are lifted.
Gradually add the granulated sugar, a tablespoon at a time, while continuing to beat the egg whites. Beat until the sugar is fully incorporated and the meringue becomes thick and glossy. The meringue should form stiff peaks when the beaters are lifted.
Place the meringue into a piping bag with a star or round tip and decorate the lemon tarts, making sure to cover the lemon curd completely.
Place the mini lemon tarts with the meringue topping in the preheated oven. Bake for about 10–15 minutes, or until the meringue is lightly browned on top.
Once baked, remove the lemon tarts from the oven and let them cool completely before serving. The meringue will set as it cools, giving it a soft and marshmallow-like texture.