In the same pot, heat the olive oil. Add the onion and cook over moderate heat until tender, about 5 minutes. Add the garlic and cook until fragrant, about 1 minute. Add the reserved clam broth, potatoes, chicken broth, thyme sprigs and bay leaf. Bring to a simmer, cover and cook over moderately low heat until the potatoes are tender, about 12 minutes. Add the chopped tomatoes to the pot, cover and simmer for 3 minutes. Discard the thyme sprigs and bay leaf. Stir in the chopped clams and season the chowder with salt and pepper. Ladle into bowls, sprinkle with the parsley and serve.