Place chopped onion in a deep saucepan with 4 tablespoons butter. Cook slowly for 2 to 4 minutes, taking care that the onion does not become brown. Add the rice and cook over medium heat, stirring constantly with a wooden spoon. After a minute or so, stir in a cup of hot beef stock in which you have dissolved the powdered saffron.
Continue cooking, adding stock as needed and stirring fr