In a clean sink, wash fresh ham well with cold water. Fill sink with fresh water and dissolve 1/2 cup coarse salt in the water. Soak ham, completely submerged for 20 minutes, before removing and drying thoroughly. Refrigerate until ready to marinate.
Place the onion, garlic, scallions, jalapeno, spices, vinegar, molasses and oil in the bowl of a large food processor and pulse until finely chopped.
Make small well-spaced slits in the skin of the ham every few inches. Rub the ham with the jerk seasoning and especially in the incisions in the skin. Be sure to use plenty of the jerk paste to ensure a boldly seasoned roast. Set the ham on a platter and marinate overnight, covered and refrigerated. Reserve any remaining jerk marinade.
Preheat oven to 325 degrees.
Place the ham in a large roasting pan, place in the oven and roast for 2 to 2 1/2 hours or until the internal temperature reads 165 degrees at its thickest part at the bone. If the ham browns too rapidly tent it with aluminum foil. Remove and let rest for 20 minutes before serving.