Tip:A long soak is the best way to ensure even cooking of the beans, but if you're short on time, you can do a quick soak in lieu of the step above: Put the beans in a large pot with enough cool water to cover by about 3 inches. Bring to a boil; boil for 2 minutes. Remove from the heat, cover, and let stand for 1 to 2 hours. Drain and rinse.
Put the beans in a heavy-duty 3-quart saucepan. Add 5 cups cool water, or enough to cover the beans by about 1 inch. Bring just to a boil over medium-high heat, reduce the heat to maintain a gentle simmer, and cook, partially covered, stirring occasionally and adding hot water if necessary to keep the beans submerged, until they begin to soften, 15 to 30 minutes
Add 1/2 tsp. salt and continue to cook, partially covered, until tender but still firm to the bite, 10 to 15 minutes. Reserve 1/2 cup of the cooking liquid, and then drain the beans in a colander set over a bowl. Rinse the beans under cold water, drain, and set aside.
While the beans cook, stir together the flour, 1/2 tsp. salt, and 1/4 tsp. pepper on a plate. Dredge the lamb in the mixture, shaking off any excess.
Heat 4 Tbs. of the oil in a 6-quart Dutch oven or other heavy-duty pot over medium-high heat. Working in 3 batches, brown the lamb on all sides, about 4 minutes per batch. Transfer each batch to a plate as it’s done. Remove the pot from the heat and let it cool briefly.
Add the remaining 2 Tbs. oil to the pot and return it to medium heat. Add the shallots and cook, stirring frequently and scraping any browned bits from the bottom of the pan with a wooden spoon or spatula, until the shallots are light brown, 8 to 10 minutes. Add the garlic, mint sprigs, tarragon sprigs, and orange zest and cook, stirring constantly, until fragrant, about 1 minute.
Stir in the broth, the tomatoes with their juice, the 1/2 cup reserved bean cooking liquid, and 2 cups water. Return the lamb to the pot and bring the mixture to a boil over medium-high heat. Reduce the heat and simmer, stirring occasionally (and more frequently toward the end), until the lamb is tender and the liquid is thickened, about 1-1/2 hours. Discard the garlic, herb sprigs, and zest.
Add the beans and cook over medium heat, stirring occasionally, until heated through, about 5 minutes. Stir 1/4 cup of the sliced mint and 1 Tbs. of the sliced tarragon into the stew. Season to taste with salt and pepper. Serve sprinkled with the remaining herbs.
The stew can be made up to (but not including) the point of adding the beans up to 2 days ahead. Tightly cover and refrigerate. Gently reheat before adding the beans, finishing, and serving.