How to Make It
Drain the beans that have been soaking overnight. In a medium saucepan, bring 5 cups of chicken stock, 2 1/2 cups of water and the speckled beans to a simmer over moderate heat. In another saucepan, bring the remaining 3 cups of stock, 1 cup of water and the black beans to a simmer over moderate heat. Cook the beans until tender, adding water when needed to keep the level of the liquid 1/2 inch above the beans. The speckled beans will take about 2 1/2 hours and the black beans about 1 hour. Season with salt. Drain the beans and put them in a large bowl.
While the dried beans are cooking, blanch the shelled fava beans in boiling salted water for 1 minute. Lift them from the boiling water with a slotted spoon, chill them in ice water and drain. Pinch the favas out of their skins. Return them to the boiling water and cook until just tender, about 4 minutes. Drain the favas and add them to the beans in the bowl.
In a medium skillet, heat 2 tablespoons of the olive oil. Add the bell peppers and cook over moderate heat until beginning to soften, 3 to 4 minutes. Add the peppers to the beans in the bowl.
Add the onion, cilantro, chervil and tarragon to the beans. Whisk the remaining 6 tablespoons of olive oil with the vinegar and mustard; season with salt and pepper. Toss the bean salad with the dressing and refrigerate until chilled. Check the seasoning before serving.