First, dice the butter and shortening and return them to the refrigerator. Pulse the flour, salt and sugar in a food processor, add the cold fat and pulse until the pieces are the size of peas.

Drizzle in the ice water with the machine running, stopping the moment the dough gathers. Turn it out onto a floured board, cut it in half, form two discs, wrap them and chill for at least 30 minutes.

Next, mash half the blueberries with 1/2 cup of the sugar in a saucepan and cook over medium heat for 15 to 20 minutes, until reduced by half. Whisk the cornstarch into the cold water, pour it in, and boil for 2 minutes until glossy and thick.

Then scrape it into a large bowl with the remaining blueberries and sugar, the sliced rhubarb, orange zest, salt, orange juice, cinnamon and nutmeg. Stir and leave it to cool to room temperature. Warm filling melts raw pastry on contact.

After that, heat the oven to 400 degrees F with a sheet pan on the middle rack. Roll the first disc out and line a deep 9.5-inch pie plate, then brush the base and sides with the blueberry jam and tip in the cooled filling.

Roll the second disc, lay it over the top and trim to about an inch of overhang. Tuck the top edge under the bottom, crimp, brush with egg wash, sprinkle with sugar and cut several vents.

Bake on the hot sheet pan at 400 degrees F for 20 minutes, then lower the oven to 375 degrees F and bake for 55 to 70 minutes more. Cover the crimped edge with a ring of foil once it is as brown as you want it.

Finally, pull it out when the juices at the vents bubble thickly and slowly rather than running thin. Cool completely on a rack, at least 4 hours, and refrigerate for an hour before slicing if you want the cleanest wedges.