In a large pot over medium-high heat, cook onion in 1/2 cup of the vegetable broth until softened, about 4 minutes. Stir in spices and cayenne and cook for 1 minute. Add remaining vegetable broth, carrots, tomatoes, corn, and potatoes. Bring to a boil, reduce heat, and simmer, covered, for 30 minutes. In a small bowl, whisk together peanut butter and boiling water. Add to pot along with cabbage and green beans. Stir well to combine and simmer for 15 minutes. Serve garnished with banana slices.