Layer Vegetables: Place onion, carrots, and celery into the bottom of an 8-quart slow cooker.
Add Beef: Position the corned beef brisket fat-side up on top of the vegetables. Sprinkle with seasoning.
Add Liquids: Pour in the beef stock followed by the beer.
Slow Cook: Cover and cook on low for 8–9 hours until beef is fork-tender.
Remove and Rest: Take out the beef and let rest. Discard or serve vegetables.
Slice and Serve: Slice against the grain and drizzle with broth before serving.
