Cut the onion, carrot, celery and green pepper in large pieces. In a food processor fitted with a steel blade chop the vegetables finely.
In a small soup pot brown the bacon lightly for 3 minutes. Add the butter and cook for 2 minutes more over low heat.
Scrape the chopped vegetables into the pot with a rubber spatula and reassemble the processor. Saute the vegetables for 5 minutes over low heat.
Sprinkle in the flour and cook the roux stirring for 3 minutes.
Pour the reserved clam liquor in a 4-cup measure and add water or clam nectar to equal 1 1/4 cup. Pour into the pot and mix with the roux.
Stir in the milk and potatoes and simmer over low heat covered for 20 minutes, stirring occasionally.
Pulse the tomatoes (I pinch off the stem end and remove any extraneous peel) with their juice and the clams in the processor for 5 seconds.
Pour the tomato-clam mixture in the pot and simmer covered over low heat for 15 minutes, stirring frequently.
Off heat, add the Worcestershire, thyme, Tabasco, salt (I use about 3/4 teaspoon) and pepper to taste.
Let the soup rest about 15 min off heat before serving.