Peel the onion and chop it finely. Rinse and drain the rice. Heat the oil in a large saucepan. Put the onion and fry it for 2-3 minutes until soft. Add the rice and fry it for 1 more minute.
Add tomato juice and 4 cups of water and bring to a boil. Put lemon grass and cook for 20 minutes or until the rice absorbs the tomato juice and water.