Pat the brisket dry and season with salt and pepper, then sear if desired.Mix cranberry sauce, broth, vinegar, brown sugar, Worcestershire, garlic, and onion soup mix.Place brisket in the slow cooker and pour the cranberry mixture over it.Cook on LOW for 8–10 hours or HIGH for 4–5 hours.Remove brisket and reduce the cooking liquid on the stovetop if you want it thicker.Slice against the grain and serve with the warm cranberry glaze.