Preheat oven to 350 degrees F. Set the roast in a roasting pan. Brush with mustard and sprinkle with the herbes de Provence. Set the onions in an overlapping layer on the top and sides of the roast; sprinkle with salt and pepper. Roast 90 to 105 minutes, or until the pork reaches an internal temperature of 160 degrees F. Transfer the roast to a plate, tent with foil, and rest 10 minutes before slicing. If desired, make gravy with pan drippings.