When you have cooked a roast of meat and tipped off the excess fat into a bowl, if it is left to harden in a cold place, you will find that a small amount of concentrated meat jelly will have gathered at the bottom. It is the addition of this to gravies and meat sauces that intensifies the taste. An onion put in the roasting tin when the meat is cooking, increases the flavour and colour of the gravy too.