Put a large saucepan over a medium flame and heat the oil. Add the onions and sauté until translucent, about 5 minutes.
Add in the tomato pure and salt. Lower the flame to low and cook for 15-20 minutes.
In a separate saucepan bring 4 cups of water to a boil. Add the rice and cook until tender, about 15-20 minutes. Scoop out the rice and add it to the sauce. Mix over a low flame for 1-2 minutes.
Serve with the grated parmigiano reggiano cheese.