Pat chops dry and coat on all sides with harissa. Transfer to a glass or ceramic 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 120 to 125°F for medium-rare, 15 to 18 minutes more. Transfer chops to a cutting board and let stand, uncovered, 10 minutes.

Serve with ground cumin for sprinkling.