In a 5- to 6-quart slow cooker, combine onion and garlic. Season brisket with salt and pepper and place, fat side up, in slow cooker. Add broth. Cover and cook on high until brisket is fork-tender, about 6 hours. Remove brisket and thinly slice against the grain. Serve with onion and some cooking liquid; sprinkle with parsley.
A whole brisket is made up of a first cut and a second cut. For this recipe, look for the leaner first cut, or "flat cut." Ask your butcher to trim the layer of fat on the brisket down to 1/4 inch, which is enough to keep the meat moist and tender as it cooks.