Remove what you have roasted from the pan and cover it to let it rest and keep it warm. Drain any excess fat from the roasting pan, and place the roasting pan on a burner over low heat.
Melt 1 tablespoon of the butter in the roasting pan, then stir in the flour and cook for half a minute. Raise the heat to medium, and whisk in the stock gradually.
Let the stock simmer until it thickens somewhat. Whisk it occasionally, scraping the bottom of the roasting pan to get any flavour bits mixed in.
Add the jelly and the port, and continue to simmer until thickened a bit more. When it has, stir in the lemon juice, then strain into a saucepan, bring it back to a simmer and whisk in the remaining butter.
Season to taste and serve hot.
Be sure the roasting pan is safe to use on a stove-top burner. Good with lamb, and with game such as duck, venison or boar.