Preheat oven to 400°F. Line 13 x 9 x 2-inch roasting pan with foil.
Mix first 4 ingredients in bowl. Rub garlic mixture all over pork. Place pork, fat side down, in prepared roasting pan. Roast pork 30 minutes. Turn roast fat side up. Roast until thermometer inserted into center of pork registers 155°F., about 25 minutes longer. Remove from oven; let stand 10 minutes.
Pour any juices from roasting pan into small saucepan; set over low heat to keep warm. Cut pork crosswise into 1/3-inch-thick slices. Arrange pork slices on platter. Pour pan juices over. Garnish with rosemary sprigs, if desired.