Heat oven to 350F.

In a saucepan, whisk sugar, cornstarch, salt and water until smooth, cook over medium heat, whisking often, until it thickens and bubbles, about 6 to 8 minutes.

Whisk 1/2 cup of the hot mixture into the egg yolks to temper, then pour the yolks back into the pan.

Cook 2 more minutes, whisking constantly, until the mixture coats the back of a spoon.

Remove from heat, stir in butter, lemon juice and zest until smooth, pour into the baked crust.

Beat egg whites and cream of tartar on medium until foamy, about 1 minute.

Increase speed, add sugar 1 tablespoon at a time, beat until stiff glossy peaks hold their shape, about 4 minutes.

Spread meringue over the hot filling, sealing it to the crust edge all the way around to prevent shrinking.

Swirl the meringue into peaks with the back of a spoon, bake 12 to 15 minutes until the peaks are golden brown.

Cool on the counter 1 hour, then chill at least 3 hours until fully set before slicing.

Slice with a knife dipped in hot water, wiping clean between cuts, for neat pieces.