Step 1

Pick over the beans and toss out any stones or unrecognizable grit. Rinse the beans thoroughly under cold running water and drain. Dump the beans in a large bowl or stock pot and add enough cold water to cover by 3 to 4 inches (7 1/2 to 10 centimeters). Soak the beans for at least 4 hours and up to overnight.

Step 2

Drain the beans and dump them into a soup pot. Add enough cold water to generously cover the beans. Bring to a boil over medium-high heat and then reduce the heat to low, cover partially, and gently simmer until the beans are tender, 1 to 1 1/2 hours.

Step 3

Drain the beans, pouring their cooking liquid into another pot or a heatproof bowl. Set aside both the beans and the liquid separately.

Step 4

Heat the olive oil in a stock pot over medium-high heat. Toss in the onion, carrot, and celery and sauté until the onion and celery are translucent, 5 to 7 minutes. Meanwhile, separate the stems from the kale leaves. Stack the kale leaves, roll them up lengthwise, and then cut the leaves crosswise into strips about 1/2 inch (12 millimeters) wide. Discard the stems or save them for another use. Toss the kale in the pot and cook, stirring frequently, until wilted, about 5 minutes. Add the garlic and sauté until fragrant, about 1 minute. Pour the tomatoes into a bowl and, using your hands, crush them into small pieces. Add the tomatoes and their juices to the pot and stir to combine.

Step 5

Measure the bean cooking liquid and add enough water to measure 4 cups (32 ounces or 1 liter). Add the beans and the cooking liquid mixture to the pot along with the bay leaf and red pepper flakes. Bring to a boil over medium-high heat, reduce the heat to medium-low, cover, and simmer just until the beans are heated through, about 10 minutes. Season to taste with salt and black pepper. Ladle the soup into warmed individual bowls and serve right away.