Season brisket on both sides with salt, pepper, garlic powder, and oregano.
Add half the onions, half the carrots, celery, 1 packet onion soup mix, and a pinch of salt and pepper to the bottom of the slow cooker. Mix to combine.
Place brisket on top and sprinkle the second packet of onion soup mix over both sides.
Top with remaining onions, carrots, mushrooms, and another pinch of salt and pepper.
Pour beef stock and red wine around the brisket.
Cover and cook 6 hours on LOW until fork-tender.
Remove brisket, tent with foil, and rest 20 minutes. Slice against the grain.
Spoon sauce over the top, finish with parsley, and serve.
