Heat the oil in a pan and fry the ginger, garlic and onion until it is golden. Then add the tomatoes, red chili powder, turmeric powder and garam masala and fry until the oil separates from the mixture. Add the lamb and cook with 2 1/ 2 cups of water for about an hour or until it is tender. Now add the potatoes and cook for 10-15 minutes until the potatoes are soft. Then add the yoghurt, sugar, salt and pepper powder and stir well. Cook for 3 more minutes. Sprinkle lime juice on top of it and garnish with chopped coriander leaves. Serve hot.