Peel sweet potatoes; cut into 2-inch pieces. In a covered large saucepan cook potatoes in enough boiling, lightly salted water to cover for 20 to 25 minutes or until tender. Drain; return to saucepan. Mash potatoeswith a potato masher or beat with an electric mixer on low speed.
In a medium microwave-safe bowl combine milk and butter; cover with vented plastic wrap. Microwave on 50% power (medium) for 1 to 1 1/2 minutes or until butter is melted, stirring once or twice. Stir in sage, mustard, and salt. Add mashed potatoes; stir or beat until light and fluffy.