The jellied substance in the bottom of your roasting pan isn't stock, it's jus - basically, the concentrated juices that are released as the proteins in the flesh are broken down. A stock is made by adding water to the carcass/flesh, along with stock vegetables, and simmering for an hour or so. If you were to make a stock, strain it, and then boil it rapidly until it has reduced by two-thirds, you would end up with a concentrated stock that jellifies when cool. This is called a demi-glace, and is pretty much the same thing that you found in the bottom of your roasting tray. Just store it in your refrigerator in an airtight container and use it in place of a stock cube; the flavour will be ten times better. You can use it in soups, stews, sauces, risottos, basically anywhere that you'd normalls use stock.