Scrub the clams well. Put into a kettle with the boiling water, cover tightly, and steam quickly till the shells are opened. Remove the clams from the shells and chop, chopping the hard part finely, leaving the soft part large. Reserve the clam liquor. In the meantime, put the potatoes, carrots and celery into a kettle with the cold water and simmer 15 minutes. Try out the pieces of pork, add the onion, and cook slowly 5 minutes. When the vegetables have cooked 15 minutes add the clams, the clam liquor, tomatoes and thyme. Cook 1 hour. Add the pork and onion and simmer 30 minutes longer. Add the pepper and more salt if necessary. Serve with pilot crackers. Note: Any desired vegetable or herb may be added.