3 sweet potatoes
4 potatoes, peeled and quartered
1 large onion, thinly sliced
2 cloves garlic, minced
1 tablespoon olive oil
1 tablespoon butter
10 cups water
1 1/2 tablespoons all-purpose flour
1/4 cup chicken broth
3/8 cup buttermilk
1 teaspoon freshly grated nutmeg
1 teaspoon ground ginger
1 teaspoon ground white pepper
1 teaspoon dried thyme
1/2 teaspoon ground cumin
salt to taste
ground black pepper to taste