Heat the oil in a soup pot and sauté the onion for 5 minutes (until the onion is translucent).
Add in the garlic and jalapeño pepper, and continue to sauté for 60 to 90 seconds.
Add the mustard and cumin and stir for 30 seconds.
Add the broth, potatoes, tomatoes and lemon juice, and bring to a quick boil and then simmer uncovered for 30 minutes.
Add the cabbage to the soup and simmer for another 5 minutes.
Salt and pepper to taste.