Heat the oil in a large pot over medium heat. Add the onion, celery, garlic, and crushed red pepper and cook, covered, stirring occasionally, until soft, about 8 minutes. Stir in the tomato paste and cook, stirring, for about 1 minute more. Tie the parsley sprigs, fresh thyme, and bay leaf together with a piece of kitchen twine and add to the pot with the potatoes. Pour in the clam juice and bring to a boil. Lower the heat and simmer, covered, until the potatoes are tender, about 10 minutes. Stir in the tomatoes and clams. Cover and bring to a low simmer. Season with pepper to taste. Divide among warm soup bowls and sprinkle with the parsley. Serve immediately.
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