Peel and finely chop the onion, put it in the bowl of the kitchen processor along with the tomatoes.
Puree until smooth.
Pour mashed potatoes into a saucepan, add broth, bring to a boil over medium heat, stirring occasionally.
Pour rice into a saucepan, reduce heat, cover the pan with a lid and cook the rice for 20-25 minutes, until soft and complete evaporation of the liquid. Season the rice with salt and pepper to taste. Serve immediately.