Preheat oven to 375°F (190°C).
In a mixing bowl, combine flour, sugar, and salt. Cut in butter until mixture resembles coarse crumbs. Add egg yolk and water, mix until dough forms. Press into tart pan. Bake for 15 minutes. Let cool.
In a saucepan, combine sugar, cornstarch, and salt. Gradually stir in water, lemon juice, and zest. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil for 1 minute. Remove from heat. Stir in butter. Gradually stir in egg yolks. Return to heat and cook for 2 more minutes. Pour into baked crust.
In a mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, beating until stiff peaks form. Spread meringue over hot filling, sealing edges to crust.
Bake in preheated oven for 10 minutes, or until meringue is golden brown. Cool on a wire rack.