Preheat oven to 400F. Scrub and pierce sweet potatoes. Bake for 1 hour or until tender.
In small saucepan, sauté onion in butter until tender. Add cranberries, syrup, water, cranberry juice and 1/8 teaspoon salt. Bring to a boil. Reduce heat; cover and simmer for 10-15 minutes or until berries pop, stirring occasionally. Stir in walnuts and mustard; heat through.
Cut potatoes in half lengthwise; sprinkle with pepper and remaining salt. Top each with 2 tablespoons cranberry mixture; sprinkle with chives and serve.