Step 1

In a large saucepan or Dutch oven, heat the butter over low heat. Add the onions, celery, green pepper and garlic. Cook, stirring frequently, for 20 minutes until they're tender.

Step 2

Add the water and potatoes, then bring the chowder to a boil. Reduce the heat and simmer the chowder, covered, until the potatoes are tender, about 15 minutes.

Step 3

Add the tomatoes, clams, salt, thyme, pepper and cayenne, and heat them through. Stir in the parsley, then ladle the chowder into bowls and serve it immediately.