Peel potatoes; cut each into 3 chunks. In a 12 cup casserole, sprinkle potatoes with 2 tbsp water; cover and microwave at High for 15 to 18 minutes or until tender,rearranging occasionally. Mash well. Stir in milk, onions and butter; microwave, uncovered at High for 1 minute or until heated through. Add salt & pepper; beat until smooth. Makes 4 to 6 servings.