How to Make It
Preheat the oven to 450°. In a heatproof bowl, cover the porcini with the boiling water and let stand until softened, about 20 minutes. Drain the mushrooms, reserving their soaking liquid, then finely chop.
Bring a large pot of salted water to a boil. Add the kale and boil until just softened, about 5 minutes. Drain and chop the kale; squeeze out any excess liquid.
Heat 1 tablespoon of the olive oil in a large, deep skillet. Add the sausage and cook over moderate heat, breaking it up with a spoon, until cooked through and lightly browned, about 5 minutes. Add the onion, leek and garlic, cover and cook, stirring, until the vegetables are softened, about 8 minutes. Add 1 tablespoon of the sage and the chopped porcini and cook 2 minutes longer. Stir in the kale, bread and cheese and transfer to a bowl to cool slightly. Season with salt and pepper and stir in the egg. Moisten the stuffing with 2 tablespoons of the reserved porcini liquid.
Set the pork on a work surface, fat side up, with the bones facing you. Using a sharp knife, run the blade along the bones, partially separating the meat but leaving 11/2 inches of it attached at the bottom. Slice through the middle of the pork, butterflying it but not cutting completely through. Open up the meat flap and season well with salt and pepper. Pack three-fourths of the stuffing into the slit in the roast and roll it back into shape. Pack the remaining stuffing between the rack and the meat. Cut nine lengths of butcher's twine and tie the roast between the bones. Place the roast in a large roasting pan, fat side up.
In a small bowl, stir the remaining 1 tablespoon of sage and 2 tablespoons of olive oil with the paprika, 1 tablespoon of salt and 2 teaspoons of pepper. Spread the paste all over the meat and roast in the lower third of the oven for 15 minutes. Reduce the oven temperature to 300° and roast for 11/2 hours longer, or until an instant-read thermometer inserted in the center of the meat registers 140°. Transfer the roast to a cutting board and let rest for 20 minutes. Remove the strings and cut the roast between the bones into individual chops.