In a small bowl, combine together corn flour and water to make corn flour slurry. Set aside.
Heat oil in a large saucepan over medium heat. Add ginger and garlic. Sauté, stirring, for 1-2 minutes or until fragrant. Add onions and green chilies. Sauté, stirring, for 2-3 minutes or until soft and translucent.
Add vegetables. Sauté, stirring, for 1 minute or until tender. Pour in vegetable stock. Add lemon juice. Stir in corn flour slurry. Cook, stirring occasionally, for about 3-4 minutes or until slightly thickened.
Add coriander (cilantro) leaves. Cook for 1 minute or until the leaves are wilted. Check seasoning. Add more salt if needed.
Transfer soup to serving bowls. Garnish with green chilies and coriander (cilantro) leaves if you like. Serve immediately.