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.
Recipe Options: Serve gravy with the pork. To do so, after the roast has been removed from the roasting pan, set the pan on the stovetop over medium-high heat. Place 2 3/4 cups beef or chicken stock in a bowl and whisk in 1/4 cup of flour. Pour this mixture into the roasting pan and simmer a few minutes, until thickened gravy forms.