Bring beans and water to a boil with onion and bay leaf. Simmer until tender. Drain well and discard bay leaf. In a food processor fitted with a metal blade, puree with walnuts, butter, chopped garlic, salt and pepper, to taste, until smooth and creamy. Mix chopped herbs together and blend half of them into the beans.
Season well and spread onto a small baking sheet, lined with plastic wrap. Cool completely.
Sprinkle generously with pomegranate seeds and remaining fresh herbs then roll into logs, using the plastic wrap to help roll. Wrap tightly and chill again.
Slice and garnish with walnuts and pomegranate seeds.