To make the cioppino base, in a medium-size heavy-bottomed pot over low heat, warm the oil. Add the onion and sauté until it just starts to soften, then add the garlic and sauté over medium-low heat for 10 minutes. Add the stock and vinegar and cook down until almost dry. Add the tomatoes, tomato paste, and oregano and simmer, uncovered, for 30 to 40 minutes. Add the salt and basil and cook for another 5 minutes. Taste and adjust the seasoning, adding red pepper flakes, if you like. Use at once, or let cool, cover and refrigerate for up to 2 days.

To prepare the cioppino for serving, divide the cioppino base among two large, heavy bottomed soup pots or wide sauté pans over medium heat. Add to each pot: 10 clams, 10 mussels, 6 ounces fish, 6 shrimp and 6 scallops, distributing them evenly over the liquid. Add the calamari, dividing it equally among the two pots, pushing it gently into the cioppino base. Cook until the shellfish have opened and all fish is opaque, 3-5 minutes; discard any shellfish that have not opened. Stir gently to mix evenly. Divide each pot of cioppino among two shallow bowls, distributing all the fish and shellfish evenly. Top each serving with an equal amount of the crab. Garnish with parsley and serve with garlic bread.