In a large, deep, nonreactive skillet, heat the olive oil. Add the scallions and cook over moderately high heat until just softened, about 3 minutes. Add the garlic and cook until fragrant, about 1 minute. Pour in the white wine, bring to a boil and boil for 1 minute. Add the sweet potatoes, tomatoes, cardmom, 1/2 tablespoon salt and 1/8 teaspoon pepper. Reduce the heat to moderate, cover tightly and cook until the potatoes are just tender, about 20 minutes.