1. In a large pot, heat oil for sautéing. Add onions and flanken and sauté for 10–15 minutes on medium heat until just golden. Add tomato and garlic and simmer for 15 minutes while the juices release from the tomato. Add remaining ingredients.
2. Simmer on medium-low heat for four to five hours or until meat is totally tender.
3. Remove meat from the pot and chop into smaller pieces.
4. If you like a smooth soup, blend the liquid in the pot with an immersion blender. Otherwise, leave as is. Return beef to the pot.