Soak the flageolets beans in cold water (at room temperature) for 12-16 hours.
Drain and rinse the beans. Put in a large pot. Add the onions, thyme, garlic and bay leaf.
Add enough water to cover the beans (the water level should be about 2 inches above the beans).
Bring to a boil, then lower the heat, cover, and let gently simmer for about 1 hour, until the beans are tender. (Add some hot water throughout if there isn't enough).
Add salt and pepper to taste.
With scissors, cut some Italian parsley leaves over the beans, mix lightly, and serve.
Note: This is one of those dishes that's even better the next day. You can keep refrigerated and reheat over low heat or even in the microwave. We usually make enough for a couple of meals, have them with pan-fried leg of lamb steaks one night, and with chicken or fish the next day.