Combine beans, tomatoes, garlic, and olive oil in a Dutch oven or heavy bottomed pot. Add enough cold water to cover the beans by 1 inch. Add the rosemary stalk. Bring the beans to a boil, then reduce the heat to medium-low and simmer partially covered. Add water as needed, to keep the beans submerged. Cook until the beans are tender, about 30-40 minutes. Season the beans with salt and pepper. Add a splash of sherry vinegar. Garnish with fresh chopped rosemary. The beans can be made 1 day ahead. Let them cool in the cooking liquid and then cover and refrigerate the beans in the pot.