Place the pie shell in an 8-inch pie plate, crimp the edges, and refrigerate for at least 30 minutes.
If you are making your own boiled cider, cook 5¼ cups of cider over medium heat until it is reduced to ¾ cup. At that point the cider will be syrupy. Skim off any foam that appears in the process.
Preheat oven to 350 degrees F.
In a large bowl, mix the brown sugar, butter and salt into the warm cider. Stir in the lemon juice. Beat the egg yolks slightly and mix in. Whip the egg whites until stiff peaks form and fold them gently into the filling mixture with a rubber spatula.
Pour the filling into the pie shell and sprinkle nutmeg on top. Bake for 40 to 50 minutes or until the filling is brown and set. The filling will still wobble when you tilt the pan, but it should not slosh. Do not reheat to serve. This is essentially cider jelly plus a meringue; reheating may make it too liquid.