Cut each of the apricots lengthwise into quarters (sixths if they're very large), discarding pits. (If using peaches, peel them and cut into thin wedges. If using plums, cut fruit into 1/2-inch slices.) You should have about 8 cups fruit total.
In a large bowl, gently mix fruit, 1/2 cup sugar, 1/4 cup flour (see notes), lemon juice, cinnamon, and salt. Taste fruit and add more sugar if desired (up to 3/4 cup total).
Line a 9-inch pie pan with a 20-inch-long sheet of foil (there should be about 5 inches of overhang on each side of the pan). Line the foil with a 20-inch-long piece of plastic wrap. Pour fruit into plastic wrap; pull edges together and fold over to seal, then repeat to seal foil over plastic. Freeze up to 3 months, removing from pie pan, if desired, after filling is hard, in about 8 hours.
From freezer to oven
To bake the pie, unwrap frozen filling and place in a 9-inch pie pan lined with an unbaked crust; cover with top crust, crimp edges to seal, and cut small slits in top to vent. Place on a foil-lined baking sheet. Bake pie in a 375° regular or convection oven until crust is browned and filling is bubbling in the center, 1 1/4 to 1 1/2 hours. (If crust gets too brown before filling is bubbling, cover edges loosely with foil.)
If baking a fresh pie, after mixing filling in step 2, pour directly into unbaked pastry, cover, crimp, and slit. Bake for about 1 hour.
Nutrition analysis per serving of double-crust apricot pie.