Heat oil in a large pot over medium heat. Add onion, bell pepper and celery; cook, stirring, until starting to soften, about 3 minutes. Add sausage and Cajun seasoning; cook, stirring, for 2 minutes. Stir in broth and tomatoes, then add cabbage; bring to a simmer. Reduce heat to maintain the simmer and cook, stirring once or twice, until the vegetables are tender, 12 to 15 minutes.