Place the sweet potatoes in a large saucepan; cover with water. Bring to a boil. Reduce heat; cover and simmer for 6-8 minutes or until tender.
Meanwhile, in a small saucepan, combine the sugar, cornstarch and salt. Gradually stir in orange juice. Bring to a boil; cook and stir for 2 minutes or until thickened. Add butter and orange peel; stir until butter is melted.
Drain sweet potatoes; place in a serving dish. Add orange juice mixture and gently stir to coat. Sprinkle with walnuts.