Preheat oven to 375°F. Between split bone and upper portion of pork loin, insert crushed garlic cloves, three sprigs of rosemary, and a quarter teaspoon each salt and pepper. Score fat side in a criss-cross cut.
Combine remaining salt and pepper, onion powder, garlic powder, and ground rosemary. Roll pork loin in mixture, patting it on gently. Using butcher’s string, tie roast at both ends and in center, securing top to bottom.
On stove top, heat olive oil and butter in roasting pan and sear fat side of pork loin.
Place roasting pan in oven and roast pork loin for one hour. Reduce temperature to 325°F and continue roasting until internal temperature at the center reaches 165°F, approximately 45 minutes.
Take out of oven and remove pork loin from roasting pan. Remove bottom bone from pork loin, place bone on a foil-covered baking sheet, and place in oven. Cover pork loin loosely with foil and let rest at room temperature.
To make the gravy, remove fat from roasting pan using a spoon, heat remaining drippings in pan on stove, then add flour to pan. Stir flour into pan juices until totally absorbed, creating a paste. Whisk in water gradually so as not to create lumps, yielding a thin mixture. Bring to a boil and cook until mixture is reduced by half and is of pleasing gravy consistency. (If pork flavor is not intense enough, add orange juice and dried cranberries to enhance flavor.)
To serve, carve loin into two-inch portions. Plate with bottom bone portion and gravy. Garnish with rosemary sprig.
