Preheat the oven to 425 degrees F. Place the pork in a roasting pan and brush with the oil. Rub the roast with the salt, pepper and sage. Roast for 20 minutes. Reduce heat to 325 degrees F and continue roasting 60 to 65 minutes more, or until the pork reaches an internal temperature of 160 degrees F when tested with an instant read thermometer. Set the roast on a plate, tent with foil and rest 10 minutes. Meanwhile, drain the excess fat from the pan and set on the stovetop over medium-high. Add the stock/flour mixture, bring to a simmer, and simmer 5 minutes. Mix in the mustard. Thinly slice the roast and serve the sauce alongside for drizzling on top.