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Preheat the oven to 425°F.
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To make the crust: Whisk together the flour and salt. Whisk together the oil and milk, and add to the dry ingredients, stirring until cohesive.
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Roll the crust to about 12" diameter on a silicone rolling mat with a piece of plastic wrap on top (between crust and rolling pin); or between two sheets of parchment or waxed paper.
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Place the crust carefully into a 9" pie pan. Press the edges gently with the tines of a fork, to flatten. Prick the bottom of the crust 6 to 8 times with a fork, to keep it from puffing as it bakes.
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Bake the crust (no need for pie weights) for 12 to 15 minutes, until it's beginning to brown. Remove it from the oven.
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Reduce the oven heat to 400°F. While the crust is baking, prepare the filling.
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To make the filling: Combine the sugar and cornstarch in a saucepan. Stir in the water.
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Cook over medium heat, stirring frequently, then constantly as it gets hotter, until the mixture thickens and comes to a boil. Boil, stirring, for 1 minute.
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Put the egg yolks in a small bowl. Stir about 1 cup of the boiling sugar mixture into the yolks.
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Return the hot yolk mixture to the saucepan. Bring to a "thick" boil (remember those burps?) and boil, stirring constantly, for 1 minute. The filling should be very thick, almost like cooled pudding or soft mashed potatoes.
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Remove the filling from the heat, and add the butter, zest, juice, and vanilla, stirring until the butter melts. Spoon the filling into the crust.
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To make the meringue: Place the egg whites and cream of tartar in a large bowl, and beat until foamy.
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With the beater going, gradually add the sugar, beating until the mixture holds a stiff peak; when you lift the beater out of the meringue, the meringue will stand straight up, with just its top curling over. Add the vanilla at the end.
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Spoon the meringue atop the hot pie filling, bringing it all the way to the edge of the pan (and covering the crust).
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Bake the pie for 8 to 10 minutes, until the meringue is as golden brown as you like it. Remove it from the oven, and allow the pie to cool for several hours before serving.
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Store the pie, covered and refrigerated for up to 3 days.
