Grind salt, oregano, caraway, cumin, marjoram and turmeric in a spice grinder or mortar until a fine powder. Add harissa, garlic and olive oil to form a thick paste. Make 8 or 9 deep slits in the pork and rub spice paste all over the surface of the joint. Cover and marinate for 1 hour.
Preheat the oven to 180 C / Gas 4.
Place the pork in a roasting tin, fat side down. Mix lemon juice and wine in a small bowl; set aside.
Roast in the preheated oven, uncovered, for 20 minutes. Turn the joint over and pour lemon mixture over the top. Add onions to the bottom of the roasting tin. Continue to roast, basting every 10 minutes with pan juices for 40 additional minutes or until internal temperature measured with a meat thermometer reaches 70 degrees C. Remove from oven and let rest about 10 minutes before slicing to serve.