1. Heat oil in a large soup pot over medium heat.
2. Add onions and one teaspoon salt. Sauté for about 10 minutes.
3. Add jalapeño, garlic, celery, and another teaspoon of salt. Sauté for an additional 10 minutes.
4. Add sweet potatoes, wine, water, and remaining two teaspoons of salt.
5. Place flanken in a net bag; add to soup. Bring soup to a boil, then reduce heat to low and simmer for four to five hours, until soup is rich and flanken is soft.
6. Remove meat from pot. Use an immersion blender to process the soup for three to four minutes, until completely smooth and creamy. Remove flanken from bone, shred, and stir into soup.
For a fancier presentation (pictured), place shredded meat on a potato chip (preferably kettle-cooked). Immediately before serving, place the potato chip on each bowl of soup. Note that it will get soggy pretty quickly, so you may prefer to serve it on the side instead.
If you prefer your soup spicy, you may leave some or all of the jalapeño seeds and membranes, which contain most of the heat. Remember to wear gloves when handling jalapeños.
Plan ahead: Soup can be frozen in an airtight container.