Heat 3 tablespoons of butter in an oven proof casserole and cook the onion over a low heat for 4 minutes. Add the rice and continue stirring for another 3 minutes. Season well. Add the saffron or coloring to the stock. Pour the stock onto the rice and mix well with a fork. Bring it to a boil.
Cover and bake in the oven at 350°F for 15 minutes until the stock is absorbed and the rice grains are separated.
Optional: Add 1 extra tablespoon of butter and stir before serving if you wish.