Use a good port, red wine or even a Madeira if that’s what you have on hand. Cranberries have so much natural pectin that this sauce will set up even if you don’t refrigerate it.
Put 1 cup port, 1 cup sugar, 1 tablespoon juniper berries, and 10 black peppercorns into a heavy saucepan and bring to a boil over medium-high heat. Add 1 bag (12 ounces) fresh cranberries and return to a boil. Reduce the heat to low and simmer until the cranberries burst and are very soft, about 10 minutes.
Strain the sauce into a bowl through a fine-mesh sieve, pushing the solids through with a rubber spatula. Transfer to a pretty serving bowl. Cover and refrigerate.