Shuck the beans and discard any that are soft or blemished. You should have about 3 cups.
Heat a 3 to 4-quart heavy pot over low heat and add the diced bacon. Once it has rendered a few tablespoons of fat, increase the heat to medium and cook until the bacon is crisp and golden brown.
Add the garlic and cook for 30 seconds. Add the onion, thyme, and bay leaf and saute, stirring occasionally with a wooden spoon, for about 8 minutes, until the onion is tender but not browned.
Add the beans and stock. Partially cover the pot and bring to a simmer. Cook the beans at a steady simmer over medium heat for 20 to 25 minutes, until they are tender. Reduce the heat to low.
Remove 2 cups of the chowder from the pot and puree in a food mill held over the pot so it falls directly back into the chowder, or puree in a food processor and return to the chowder. Add the tomatoes and their juices, along with the molasses, and let the chowder simmer slowly for another 15 minutes. Remove from the heat and season with salt and pepper. If you are not serving the chowder within the hour, let it cool a bit, then refrigerate; cover the chowder after it has chilled completely. Otherwise, let it sit at room temperature for up to an hour, allowing the flavors to meld.
When ready to serve, reheat the chowder over low heat, stirring occasionally so it doesn't stick. Ladle the chowder into cups and sprinkle with the slice scallions.
Variation Dried Bean Chowder:
Substitute 8 ounces dried shell beans, limas, or other white bean for the fresh shell beans. Soak them overnight in enough water to cover generously; drain. Depending on the bean you choose, the cooking time may be doubled or even longer. Follow the recipe and simmer until the beans are tender, adding water as necessary (about 1 cup) to compensate for the evaporation during the longer cooking time.
Variation Cider and Bean Chowder:
This idea also comes from the Shakers, who used sweet apple cider as the liquid in several of their soups. It adds a new dimension to the sweet-and-sour flavor of this chowder. Cider is pressed from late August through November, so you can try this in either the fresh or dried bean versions. Substitute 3 cups apple cider for 3 cups of the chicken stock (you will still use 2 cups of chicken stock).