Melt the butter in a heavy 3 litre saucepan over moderate heat. Add the onion, and cook, uncovered, for 5 minutes, or until soft.
Stir in the rice, red pepper, chilli powder, origanum, tomato juice and tomato puree, then add the water and salt. Stir once, cover and bring to a boil. Reduce the heat and simmer for 20 minutes, or until the rice is tender, and the liquid has been absorbed.