Peel and quarter potatoes. In a covered large saucepan, cook potatoes for 20 to 25 minutes or until tender in enough boiling, lightly salted water to cover; drain.
Mash with a potato masher or ricer or beat with an electric mixer on low speed. Add 1/3 cup of the half-and-half and the 1/4 cup butter, stiring until nearly smooth. Stir in yogurt, the 1/2 teaspoon salt, and 1/4 teaspoon pepper. If necessary, gradually mash in enough of the remaining 1/3 cup half-and-half to make mixture light and fluffy. Stir in green onion.
For mushroom gravy, in a medium saucepan, heat the 3 tablespoons butter over medium heat. Add onion and garlic; cook and stir for 1 minute. Add mushrooms; cook just until mushrooms are tender, stirring occasionally. Stir in broth, madeira, tarragon, 1/4 teaspoon salt, and 1/4 teaspoon pepper. Cook about 3 minutes or until mushrooms are tender, stirring occasionally. Stir in whipping cream. Simmer, uncovered, for 2 minutes more.
To serve, spoon mashed potatoes into a serving bowl. Make a depression in the center; spoon mushroom gravy over potatoes. Makes 8 servings.