Heat olive oil in a deep pan. Add onion and sauté for a minute. Add ginger, garlic and green chillies. Sauté on low heat and add turmeric powder, salt, roasted cumin powder and drained dal. Add five cups of water. Bring to a boil and cook, on medium heat, for ten to fifteen mintues or till dal is very soft. Pass through a soup strainer. Bring strained dal to a boil and add water to adjust consistency. Continue to simmer for two to three minutes. Add lemon juice and mix. Serve piping hot garnished with coriander leaves.