Put the onion and tomatoes in a food processor and process to a smooth puree. Scrape the puree into a saucepan, pour in the stock, and bring to a boil over medium heat, stirring occasionally.

Add the rice and stir once, then reduce the heat, cover, and simmer for 20-25 minutes, until all the liquid has been absorbed and the rice is tender. Season with salt and pepper and serve immediately.