Preheat the oven to 350 degrees F (175 degrees C). Arrange tart shells on a baking sheet.

Bake in the preheated oven until heated through, about 5 minutes. Remove from the oven; leave oven on.

Whisk egg yolks, 3/4 cup sugar, butter, lemon juice, and zest together in a medium bowl. Whisk in buttermilk. Add flour and salt; whisk well.

Pour filling into the tart shells, filling them almost to the top.

Bake in the preheated oven until filling rises and appears set, about 15 minutes. Watch carefully to avoid overcooking.

Prepare meringue while the tarts are baking. Beat egg whites with an electric mixer on high speed until frothy. Add remaining 1/2 cup sugar and cream of tartar; continue beating until meringue is stiff and shiny.

Top baked tarts with meringue. Ensure that you press the meringue down to form a seal around the edges.

Return tarts to oven and bake until meringue starts to brown, about 10 minutes more. Let cool before serving.