In a 4-quart Dutch oven cook potatoes with 1/2 teaspoon salt, covered, in enough boiling water to cover for 20 minutes or until tender.
Meanwhile, in a small saucepan cook onion and garlic in hot butter until onion is tender. Stir in wine. Bring mixture to boiling; reduce heat. Simmer, uncovered, for 3 to 4 minutes or until reduced to 1/2 cup.
Drain potatoes. Mash with a potato masher or beat with an electric mixer on low speed. Add wine mixture, 1/2 teaspoon salt, and 1/4 teaspoon back pepper. Gradually beat in whipping cream to make potato mixture light and fluffy. Fold in blue cheese. Top with green onion. Makes 10 (1/3-cup) servings.