Preheat the oven to 275 degrees. Set a large rack in a very large roasting pan. In a small bowl, combine the garlic salt with the sweet paprika, 1 tablespoon of kosher salt , and 2 tablespoons of black pepper. Stir in 1/4 cup of the canola oil. Rub the spice paste on both sides of the short ribs and transfer to the rack in the roasting pan, meaty side up. Roast for 4 hours, until the meat is tender, and pulls away from the bones. Transfer the short ribs to a platter and pour off the fat in the roasting pan. Increase the oven temperature to 325 degrees.
Meanwhile, in a medium heatproof bowl, soak the porcini in the boiling water until they are softened, about 20 minutes. Drain the porcini and reserve 2 cups of the soaking liquid. Rinse and coarsely chop the porcini mushrooms .
Place the roasting pan over a burner and add the olive oil . Add the onion, carrots, celery, garlic, white mushrooms, and chopped porcini mushrooms and cook over moderately high heat, stirring, until the vegetables are softened and just beginning to brown, about 10 minutes. Add the wine and cook, scraping up any browned bits stuck to the bottom of the pan, until the wine has evaporated, about 5 minutes. Add the beef stock, the reserved 2 cups of porcini mushroom soaking liquid, and 3 cups of water and bring to a boil. Season with salt and pepper.
Return the short ribs to the roasting pan, meaty side up. Cover the pan very tightly with foil and braise in the oven for 1 hour and 15 minutes, until the meat is tender. Remove the foil and spoon off as much fat as possible from the pan juices. Transfer the short ribs to a platter.
Meanwhile, in a large skillet, heat the remaining 1/4 cup of canola oil . Add the matzo farfel and cook over moderately high heat, stirring, until lightly toasted, about 5 minutes. Stir the toasted farfel into the pan juices in the roasting pan.
Return the short ribs to the pan, nestling them into the liquid and roast, uncovered, for 15 minutes, until the matzo farfel is moistened and plump.
Transfer the short ribs to a cutting board. Remove the bones and slice the meat across the grain. Stir the spinach into the farfel, season with salt and pepper and let wilt. Spoon the farfel onto a platter and top with the meat. Sprinkle the parsley and coarse sea salt on top and serve.
MAKE AHEAD The recipe can be prepared through Step 4 and refrigerated for up to 2 days. Slice the meat cold and rewarm, covered, in a 325 degrees oven.
NOTES Matzo farfel is available in the kosher section of supermarkets during Passover. To make this dish without observing the holiday's dietary restrictions, substitute pearled barley . Stir 1 1/2 cups of pearled barley into the sauteed vegetables in Step 3 and toast for 2 to 3 minutes before adding the wine, then proceed with the recipe (eliminating Step 5 altogether).
