In a large dutch oven, heat the salt pork until the fat melts. Add the onions. Cook ocher gentle heat until very soft. Add the clam juice, potatoes, seasonings, tomato puree and a little water. Simmer until the potatoes are soft, then add the quahogs. Heat and taste for seasoning. Add water if needed. It is best to use old, not new, potatoes, because they thicken the chowder with their starch. Crush some pilot crackers and stir them into the chowder to thicken it further, near the end of the cooking. This recipe makes about 20 8oz servings. Enjoy!