Preheat the oven to 400 degrees . Season the lamb chops with salt and pepper; let stand for 5 minutes. In a small bowl, stir together the parsley and fresh and dried thyme; press onto the lamb chops.
In a large ovenproof skillet, heat the oil until smoking; add the lamb chops and cook for 2 minutes on each side and 1 minute along the edge. Dot the chops with the butter and transfer the pan to the oven. Roast, turning once and basting with the pan juices, until an instant-read thermometer registers 125 degrees , about 8 minutes. Remove from the oven and transfer the chops to a cutting board; let stand for 5 minutes. Serve with the grainy mustard.