In a bowl, stir together the flour and salt, then cut in cold shortening with a pastry blender until it looks like small peas. Add cold water one tablespoon at a time, just until it holds together, then chill 45 minutes.
On a floured counter, roll the chilled dough into a circle about two inches wider than the pie plate. Ease it into the plate, press the bottom and sides, then trim and flute the edge.
Prick the bottom and sides with a fork so steam escapes, then bake at 450°F for about 14 minutes, until light golden brown.
The edges should look dry and pale gold, not deep brown. A crust that puffs badly usually needed more fork holes before baking.
In a saucepan, whisk sugar and cornstarch together first, then stir in water slowly to avoid lumps. Over medium heat, this thickens and starts to bubble, taking around 8 to 9 minutes with steady stirring.
Whisk the yolks in a small bowl, then slowly pour in half the hot sugar mixture while whisking fast. Stir this back into the saucepan and boil two more minutes.
A grainy or lumpy filling means the yolks heated too fast. Strain the filling through a fine mesh sieve to smooth it out again.
Off the heat, stir in butter, lemon zest, and lemon juice with a whisk, until the filling turns glossy and smells sharp and bright. Keep it warm and covered while the meringue comes together.
In a clean bowl, beat egg whites and cream of tartar until foamy, then add sugar slowly, one tablespoon at a time, on high speed. Keep beating until peaks turn stiff, glossy, and hold their shape.
Pour the hot filling straight into the baked crust, then spread meringue over the top right away, sealing it to the crust edge. Bake at 350°F for 20 to 25 minutes, until golden brown on top.
Cool the pie on a rack for one hour, then chill in the fridge for about 4 hours, until the filling sets firm enough to slice cleanly.
