Soak the beans in water overnight. Drain, then transfer to a Dutch oven. Add 10 cups water, 2 sage leaves, the smashed garlic and 1 tablespoon salt. Bring to a boil over high heat, then reduce the heat to medium low; cook until the beans are tender, about 1 hour. Drain, reserving the liquid. Discard the garlic and sage.
Meanwhile, preheat a grill to high or preheat the oven to 475 degrees F.
Return the beans to the Dutch oven. Add the sliced garlic, olive oil, the remaining 4 sage leaves, the tomato, 1 1/2 cups of the reserved cooking liquid, 1/2 teaspoon salt and 1/4 teaspoon pepper. Place the pot, uncovered, on the grill or in the oven; cook until creamy, 15 to 20 minutes. Add the parsley and more cooking liquid, if necessary. Season with salt and pepper and drizzle with olive oil.