In a large cast-iron or other heavy skillet, heat oils over medium-high heat. Add sweet potatoes, onion, pepper and gingerroot; cook and stir 5 minutes. Add water. Reduce heat to low; cook, covered, until potatoes are tender, 8-10 minutes, stirring occasionally., Stir in next 5 ingredients; cook and stir over medium-high heat until heated through, about 2 minutes. Top servings with cilantro and, if desired, chopped macadamia nuts.