Step 1Place the beans in a bowl and cover by 3 inches with cold water. Soak 12 to 14 hours. Drain. Place the beans in a heavy-bottomed pot and cover by 2 inches with water. Add the bouquet garni and garlic, and bring to boil, reduce the heat and simmer until the beans are tender, 1 to 1 1/2 hours. Toward the end of cooking, finely chop the onion, carrot and leek. Heat 2 tablespoons of oil in a skillet over medium heat. Add the vegetables and cook until the onions turn golden, 5 minutes.
Step 2Remove the bouquet garni, add the onion, carrot and leek. Simmer until the beans are very tender, 30 minutes. Season with salt and pepper.
Step 3Drain off 3/4 (about 2 cups) of the liquid and reduce in a saucepan over medium heat until it reaches a medium-thick consistency, 5 minutes. Return the liquid to the beans, toss and add the parsley sauce. Toss and serve immediately.
Step 1Combine the parsley, butter and garlic in a food processor and pulse until finely chopped, about 15 seconds.