How to Make It
Place allspice berries, bay leaves, peppercorns, mustard seeds, and coriander seeds in center of 6- x 6-inch cheesecloth square; gather into a pouch, and tie with a piece of kitchen twine. Set aside.
Spray inside of a 6-quart slow cooker with cooking spray. Place onion in bottom of slow cooker. Place brisket on top of onions, and sprinkle evenly with smashed garlic cloves and 1 teaspoon of the salt. Pour broth over top of brisket. Add the spice pouch to the slow cooker. Cover and cook on LOW 4 hours.
Add potatoes, carrots, pepper, and remaining 1 teaspoon salt to slow cooker. Cover and cook on LOW 2 hours. Nestle cabbage wedges into mixture; cover and cook on LOW 2 more hours.
Remove beef from slow cooker, and let rest 10 minutes before slicing. Remove vegetables from slow cooker with a slotted spoon. Remove and discard spice pouch. Serve corned beef on platter with vegetables. Spoon a little of the cooking liquid over corned beef and vegetables, and serve some of the au jus on the side.