STEP 1Heat the oil in a large saucepan over medium heat. Add the onion, season with 3/4 teaspoon salt and 1/4 teaspoon pepper and cook, covered, stirring occasionally until tender, 8 to 10 minutes.
STEP 2Stir in the garlic and ginger and cook, stirring for 1 minute. Stir in the cumin and cayenne and cook 1 minute more. Add the sweet potatoes and mix to combine.
STEP 3Add the tomatoes, peanut butter, and 4 cups water and bring to a boil. Reduce heat and simmer, covered, until the sweet potatoes are tender, 18 to 20 minutes. Serve with cilantro and peanuts, if desired.