Cut salt pork into small pieces and saute slowly in soup kettle. Remove pork and add onions to grease, cooking slowly until onions soften. Add potatoes, water, salt, and bay leaf. Bring to boil, reduce heat, and simmer about 10 minutes. Add tomatoes and cook until potatoes are done. Add clams, thyme (start with ½ teaspoon), fennel, and salt and pepper. Simmer for a few minutes and serve hot.