This is, for me, one of the great classic English sauces, provided it's made with a good-quality redcurrant jelly with a high fruit content; some of the commercial varieties are lacking in fruit and are too sickly sweet.
Cumberland sauce is always served cold and is a wonderful accompaniment to either hot or cold gammon, tongue, cold goose or game, and it goes extremely well with a slice of Old-fashioned Raised Game Pie, see below. This sauce should not be thickened – it is meant to have a thinnish consistency.
First, thinly pare off the zest of both the lemon and the orange, using a potato peeler, then cut them into very small strips ½ inch (1 cm) long and as thin as possible.
Boil them in water for 5 minutes to extract any bitterness, then drain well. Now place the redcurrant jelly in a saucepan with the port and melt, whisking them together over a low heat for about 5 or 10 minutes. The redcurrant jelly won't melt completely, so it's best to sieve it afterwards to get rid of any obstinate little globules.
In a serving bowl, mix the mustard and ginger with the juice of half the lemon until smooth, then add the juice of the whole orange, the port and redcurrant mixture, and finally the strips of lemon and orange zest. Mix well and the sauce is ready to use.
Cumberland sauce stores well in a screw-top jar in the refrigerator for up to two weeks.