Preheat the oven to 425 degrees F. Place the pork in a roasting pan. Brush with the mustard; season with sage, salt and pepper. Roast 15 minutes. Reduce heat to 325 degrees F; roast 55 minutes more, or until the centre registers 160 degrees F (71 degrees C) on an instant read meat thermometer. Place the roast on a plate, tent with foil and allow it to rest 10 minutes. Drain the excess fat from the roasting pan before placing it on the stovetop over medium-high heat. Add the stock/flour mixture, bring to a simmer, and simmer 5 minutes. Thinly slice the pork and stuff into the buns with the onion. Divide the bunwiches among plates and serve with small bowls of gravy alongside for dipping.