Scrub the clam shells, rinse thoroughly in cold water, and place in a large pot. Add cold water to cover and bring to a boil over high heat. Cook until the shells open, about 5 minutes. Using a slotted spoon, transfer the clams to a large bowl. Discard any clams that have not opened.
Strain the cooking liquid through a chinois or a strainer lined with a coffee filter, and reserve ¾ cup of this liquid as clam broth. Return the clams to the pot, add cold water, and stir to remove any remaining sand. Drain and reserve.
Heat the oil in a large saucepan over medium flame and sauté the garlic halves until golden, about 2 minutes. With a slotted spoon or tongs, remove and discard the garlic. Add the onions to the saucepan and sauté for 3 to 4 minutes, until soft and translucent. Coarsely chopped the tomatoes and add with their juice to the saucepan and bring to a boil. Reduce the heat to low, cover, and simmer for 25 minutes, stirring occasionally.
Season to taste with salt and pepper. Stir in the tomato paste and add the basil and parsley. Simmer uncovered for 5 minutes.
Add the reserved clam broth and clams to the sauce and bring to a boil. Cover, reduce the heat to low, simmer for 8 to 10 minutes, or until the clams are heated through. Spoon the clams and sauce into a large serving bowl, garnish with parsley and serve immediately.
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