Pat the roast dry and season all sides with salt, pepper, and celery seed.
Layer sliced onions and celery pieces in the bottom of the slow cooker, then add smashed garlic cloves.
Place the seasoned roast on top of the vegetable layer.
Whisk together beef broth, red wine vinegar, flour, and dried parsley until smooth, then pour over the roast.
Cover and cook on low for 8 hours or high for 4-5 hours until the meat shreds easily with a fork.
Remove roast and let rest briefly, then slice or shred and serve with the celery, onions, and pan gravy spooned over top.