How to Make It
Preheat the oven to 350°. In a food processor, pulse the truffles and any liquid in the jar until finely chopped and spreadable.
Lay the pork on a work surface with the skin side facing down. Season with salt. Spread the truffle puree evenly over the meat and scatter the sage on top. Starting at a short edge, roll the pork belly into a log. Tie tightly at 1-inch intervals with kitchen twine. Season all over with salt.
Set the pork seam side down in a roasting pan and scatter the potatoes around it. Drizzle the pork and potatoes with the oil. Season the potatoes with salt and pepper and toss to coat. Arrange the potatoes cut side down.
Roast until the potatoes are browned and tender, about 1 hour. Transfer the potatoes to a plate and continue to roast the pork for about 20 minutes, until an instant-read thermometer inserted in the center registers 140° to 145°. If the fat is not crisp, increase the oven temperature to 450° and roast for 5 minutes longer, until crisp. Turn off the oven.
Transfer the pork to a work surface and let stand for 15 minutes. Put the potatoes back in the roasting pan and return to the oven to warm.
Remove the twine from the pork and slice the roast crosswise. Arrange the meat on a platter and pour any juices from the cutting board over it. Serve with the potatoes.