Whisk together zest, lemon juice, sugar and eggs in a heavy 2-quart saucepan.
Add butter all at once and cook over moderately low heat, whisking constantly, until curd is thick enough to hold marks of whisk and first bubbles appear on surface, about 10 minutes.
Immediately pour curd through a fine sieve into a bowl, then chill, covered.
Beat whites with cream of tartar using an electric mixer at medium speed until they form soft peaks.
Increase speed to high and add sugar, 1 tablespoon at a time, beating until meringue just forms stiff, glossy peaks.
Spoon cold lemon curd into a large zip-lock bag and remove any air pockets; knead slightly.
Cut one corner tip off the bag to allow you to squeeze the cold curd onto the toast. Gently squeeze the lemon curd bag onto the toast making sure to have a smooth, flat top.
Add a dollop of meringue decoratively over the filling. Immediately bake until meringue is golden-brown, 8-10 minutes.
