Heat oil in a wok or large skillet that has a cover. Add orange zest, garlic, ginger, and chili paste and stir-fry until fragrant, about 10 seconds. Add sweet potatoes and stir-fry until coated, about 30 seconds.
Re-stir sauce and add to sweet potatoes. Bring to a boil, then cook, covered, over moderate heat until sweet potatoes are tender but not soft, 10 - 12 minutes.
Dissolve cornstarch in water and add to sweet potatoes, stirring until thickened. Stir in scallions.