1. Dissolve 3 tablespoons salt in 4 quarts cold water in large bowl or container. Add beans and soak at room temperature for at least 8 hours and up to 24 hours. Drain and rinse well.

2. Adjust oven rack to lower-middle position and heat oven to 250 degrees. Heat oil and pancetta in large Dutch oven over medium heat. Cook, stirring occasionally, until pancetta is lightly browned and fat has rendered, 6 to 10 minutes. Add onion, celery, and carrots. Cook, stirring occasionally, until vegetables are softened and lightly browned, 10 to 16 minutes. Stir in garlic and cook until fragrant, about 1 minute. Stir in broth, water, bay leaves, and soaked beans. Increase heat to high and bring to simmer. Cover pot, transfer to oven, and cook until beans are almost tender (very center of beans will still be firm), 45 minutes to 1 hour.

3. Remove pot from oven and stir in greens and tomatoes. Return pot to oven and continue to cook until beans and greens are fully tender, 30 to 40 minutes longer.

4. Remove pot from oven and submerge rosemary sprig in stew. Cover and let stand 15 minutes. Discard bay leaves and rosemary sprig and season stew with salt and pepper to taste. If desired, use back of spoon to press some beans against side of pot to thicken stew. Serve over toasted bread, if desired, and drizzle with olive oil.