Heat oil in heavy large saucepan over medium-high heat. Add onion and chili; saute until tender, about 5 minutes. Stir in ginger and next 4 ingredients and cook 1 minute. Mix in 7 cups water, sweet potato and peas. Bring mixture to boil. Reduce heat to medium-low, cover and simmer until potato and split peas are very tender, about 1-1/2 hours.
Whisk soup vigorously for smoother texture. Thin with additional water if necessary. Season with salt and pepper. (Can be prepared 1 day ahead. Refrigerate.) Return soup to simmer. Ladle soup into bowls. Top each with 1 tablespoon yogurt, lime slice and cilantro and serve.
Per Serving: calories, 254.97; fat, 3.18 g; sodium, 26.07 mg; cholesterol, 28 mg
Source: The Moosewood Collective, Ithaca NY (Bon Appetit, Jan.1992) Typed for you by Karen Mintzias