Preparation:
In a large saucepan cook the onion and mushrooms in hot oil until onion is tender but not brown. Add the potatoes and broth. Bring to a boil. Reduce heat. Cover and simmer about 30 minutes or until potatoes are tender. Stir in beans. In a small bowl, stir together the buttermilk and cornstarch. Stir into potato mixture. Cook and stir till thickened and bubbly. Cook and stir for 2 minutes more. Stir in 1 tbsp snipped basil. To serve, ladle soup into individual bowls. Dollop each serving with yogurt or sour cream. If desired, garnish with fresh basil.