Heat butter or oil in a large pot over medium-high heat. Add onion, garlic, bell peppers and celery and stir. Cook for 3 to 4 minutes. Stir in soaked beans, then add chicken broth, ham hock, Cajun seasoning, salt, pepper, thyme or bay leaves, and cayenne, if using. Bring to a boil, then reduce heat and cover for 30 minutes.