Preheat a grill pan over medium heat. Sprinkle the pork with the olive oil and some salt and pepper, and then grill for 7 to 10 minutes on each side. In a small pot, add the apple cider, maple syrup, vinegar and rosemary. Simmer over low heat until the mixture thickens, about 20 minutes. Remove the pork from the grill and rest it for 10 minutes before slicing. Serve with the apple cider sauce.