Step 1Soak beans in water in large pot overnight.
Step 2Bring beans and water to a boil, add chorizo, then reduce heat and simmer, covered, until beans are barely tender, about 25 to 30 minutes. Stir occasionally to prevent beans from sticking to bottom of pot.
Step 3Add kale, potatoes, salt to taste and red pepper flakes. Return to simmer and cook until potatoes are tender, about 25 minutes. Soup is best when allowed to sit for several hours. (Reheat before serving.)