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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. Cover and refrigerate any leftover pie.
