Roll out the two disks of dough for the crusts. Ease one into a 9-inch pan; trim so the edges extend just beyond the pie tin. Put this in the refrigerator, along with the rolled-out top crust, to keep cool while you make the pie filling.

Put the cut-up rhubarb in a large bowl. Sprinkle the lemon juice or apple cider vinegar over it. Measure out the dry ingredients and heap them on top of the fruit, but don't mix them until you are ready to assemble and bake the pie.

When you are ready to bake the pie, give the filling a stir. If the dry ingredients don't cling to the fruit, and fall instead into a heap at the bottom of the bowl, wet your fingers with water so you can shake droplets over the mixture just until it loses its powdery texture and takes on a damp-sandy look. Don't add any more water than is necessary to reach this stage; just enough to get the sugar/flour mixture to cling evenly to the rhubarb.

Take your crusts out of the refrigerator. Spoon the fruit filling into the bottom crust. Lay the top crust on the fruit. Trim, roll and crimp the edges of the pie. Make a few decorative vents in the top crust.

Preheat the oven to 425. Put the pie in the freezer while the oven is coming fully up to temperature, about 20 minutes. When the oven is preheated, take the pie out and brush it lightly with beaten egg white. Sprinkle it with sugar.

Bake at 425 for 15 minutes. Reduce oven heat to 375 and bake another 45 minutes or so, until the pastry is deep golden brown and you can hear the fruit filling bubbling inside.

Allow the pie to cool completely to room temperature -- probably 6 to 8 hours -- before slicing. It will hold together better this way.