In a large pot, sauté the onion in the olive oil, over medium heat until the onion is soft and translucent, approximately 5 minutes.
Add the trahana to the pot, and mix well, coating the grains with oil.
Add the tomato juice and water and bring to a boil, stirring very often. Once you have reached a boil, reduce heat to medium low and continue to cook, stirring very frequently, until the trahana is tender, approximately 45 minutes. If your trahana appears too thick, add more water, 1/4 cup (60 ml) at a time.