Place the beans in a colander and rinse with cold water. Pick over the beans to remove any stones. Place the beans in a clean pot, cover with fresh cold water and soak for at least 4 hours or overnight. Drain.
Place the beans in a large pot with 2 quarts of water and bring to a boil.
Reduce the heat and simmer until the beans are tender, about 1 hour. Drain and cool to room temperature.
Using a food processor, pulverize the walnuts. Then add the garlic, vinegar, oil and 3 tablespoons of the cooking liquid from the beans, enough to make a smooth dressing.
Season the dressing with salt, pepper and cayenne to taste. Stir in half of the scallions and half of the herbs.
Pour the dressing over the beans and garnish with the remaining scallions and herbs. Serve at once or toss and chill until ready to serve. Let the beans come to room temperature before serving.