Place the cranberries and onion in the bowl of a food processor and pulse until coarsely chopped, about 14 to 15 pulses. Scrape the sides down once after about 10 pulses.
Transfer the chopped cranberry mixture to a mixing bowl, add the remaining ingredients, and mix to thoroughly combine. Transfer the relish to a plastic container and freeze.
Early Thanksgiving morning (or whichever day you plan to eat this), move the container from the freezer to the refrigerator to thaw. If you need to expedite the thawing process, allow the container to sit at room temperature for an hour or two, but here’s the key according to Mama Stamberg: the relish should still have some little icy slivers when served. Stir before transferring to a serving dish and break up any sizeable frozen clusters with a dinner fork.