Meanwhile, in large nonstick skillet, heat oil over medium heat; cook green peppers, onions, garlic, chili powder, cumin, coriander, 1/2 ts salt and 1/4 ts pepper, stirring, for 10 minutes. Stir in tomatoes and black beans; bring to boil. Reduce heat and simmer, covered, for 20 minutes. Stir in corn and 2 tb of the chopped coriander, cook, uncovered, for 5 minutes. Place sweet potatoes on plates; slit lengthwise through middle and mash inside slightly with fork. Season with salt and pepper to taste. Using back of spoon, make hollows in sweet potatoes and spoon in chili mixture. Top with spoonful of yogurt and sprinkle with remaining chopped coriander. Garnish with coriander leaves and lime.