To make the crust: Have all of your ingredients ready and chilled. Combine the flour, salt and sugar in a food processor. Add the butter and lard. Use the food processor to work the butter and lard into the flour. Work the fats into the flour until it starts to stick together when squeezed, but you want to keep some of the fat in pea-size pieces. If it is too soft and it won't stay in pieces, place the bowl in the freezer for about 15 minutes and then try again.
Whisk the egg, add it along with the water and vinegar to the flour/lard mixture. Stir this with your hand or a wooden spoon, but be very gentle. The goal is to keep the pea-size chunks of fat intact. It will start to look shaggy and not dry. It should hold together when you squeeze it in your hand. Gently knead the dough into a ball. Divide the dough into two pieces and create round disks. Wrap the dough in plastic and refrigerate for at least an hour, or overnight.
Preheat oven to 425 degrees F with the rack on the bottom shelf.
In a large bowl, toss together the rhubarb, cherries, sugar, cornstarch and spices. Roll out one of the circles of dough and drape it into the pie plate. Place the filling in the pie shell. Break the butter up into several pieces and put it over the filling.
Weave the lattice over the top. Crimp the edges. Brush the top with egg wash.
Sprinkle generously with sugar.
Place the pie on a baking sheet, just in case the juices bubble over.
Bake for 30 minutes. Reduce the heat to 350 degrees F, bake until the juices are bubbling and translucent, and the fruit is very soft. This can take up to 1 1/2 hours. If the top crust starts to get too much color, tent the pie loosely with foil.
Allow the pie to cool so the juices set enough to cut the pie.
Enjoy with ice cream.