Heat oil in a large pot over medium heat. Add onion, fennel, peppers, and garlic and cook for 12-15 minutes, until vegetables are soft but not browned.
Stir in chipotle peppers, oregano, chili powder, and salt and cook for an additional one minute.
Add remaining ingredients and gently simmer over low heat for at least an hour, stirring occasionally. Garnish with shredded cheese if desired and serve.