Steam the kale in a steamer basket set over boiling water until tender, about 5 minutes. Drain well, pressing out any remaining liquid, and set aside.
Heat the olive oil in a large pot over medium heat. Add the onion and cook until softened, about 5 minutes. Add the garlic, bell pepper and sweet potato. Cover and cook until softened, about 5 minutes. Stir in the ginger, cumin and cayenne if using, and cook for 30 seconds. Stir in the broth, the tomatoes and their juices and salt to taste. Bring to a boil, then reduce the heat to low, add the kidney beans, and simmer until the vegetables are soft, about 30 minutes.
In a small bowl, thin out the peanut butter with 1 cup of the hot liquid from the stew, stirring until blended, then stir the mixture back into the pot. Add the cooked kale, stirring to incorporate. Serve hot, sprinkled with the roasted peanuts.