Pat chops dry and coat on all sides with harissa. Transfer to a glass dish and marinate, covered and chilled, 2 hours.
Sprinkle chops with salt. Heat oil in a 12-inch heavy skillet over moderate heat until hot but not smoking, then brown chops on all sides, 7 to 8 minutes total.
Add butter and thyme and continue to cook, turning chops frequently with tongs and basting occasionally, until thermometer inserted between ribs to center of a chop (do not touch bone) registers 130 to 135°F for medium-rare, 5 to 8 minutes more. Transfer chops to a cutting board and let stand, uncovered, 10 minutes.
Serve with ground cumin for sprinkling.