Spray the bottom, sides and rim of a 9-inch glass pie plate with nonstick cooking spray. Wipe away any excess oil with a paper towel.
Prepare the meringue shell: In the bowl of an electric mixer fitted with the whisk attachment, combine the egg whites, cream of tartar and 1/4 teaspoon salt. Begin whipping the whites on medium speed until they start to become opaque and hold soft peaks, about 2 minutes. With the mixer running, spoon in the sugar, 1 tablespoon at a time — don't rush this stage, as you want the sugar to dissolve as much as possible to yield a sturdy shell.
When all the sugar has been added, increase the mixer speed to medium-high and beat until the meringue reaches very stiff, glossy peaks, about 1 minute more. The meringue should ball up in the whisk and create an almost vertical peak when you lift the whisk from the bowl. Beat in the vanilla and almond extracts.
Pile the meringue into the prepared pie plate. Using the back of a small spoon, shape the cloud of meringue into a thick-bottomed pie shell, so it billows up the sides about an inch and clings to the inner rim of the edge of the plate. Smooth the bottom evenly. Bake the shell until crisp, set and ivory in color, about 1 1/2 hours. Prop the oven door open with a wooden spoon and allow the crust to cool completely in the oven, about 1 hour more; don't worry if the shell falls or cracks a bit while cooling.
Meanwhile, prepare the lemon filling: In a small cup, combine 2 tablespoons of the lemon juice with the gelatin and stir until any lumps disappear. Allow the gelatin to soften for 5 to 10 minutes.
In a 2- to 2 1/2-quart saucepan, whisk together the remaining lemon juice with the sugar, egg yolks, eggs, lemon zest, and salt. Set the pan over medium heat. Whisking often, cook the curd until a few lavalike steam bubbles come to the surface, about 5 minutes. Whisk in the lump of softened gelatin until melted. Remove the pan from the heat and whisk in the butter. Pour the curd through a sieve into a clean bowl and cover the surface with plastic wrap. Allow the curd to cool at room temperature while the meringue shell is cooling,