Prepare the standing rib roast by letting it sit at room temperature for 30-60 minutes. Score the top fat in a diamond pattern and season generously with salt, pepper, and dried thyme.

In a skillet, heat vegetable oil over medium-high heat. Sear the rib roast on all sides until golden-brown, about 3-4 minutes per side. Remove and set aside.

Sauté sliced onion in the same skillet until golden brown, about 5-7 minutes. Transfer caramelized onions to the slow cooker.

Deglaze the pan with red wine, stirring to scrape up browned bits. Simmer for 2-3 minutes, then pour over the onions in the slow cooker.

Add bay leaves, fresh rosemary, and beef broth to the slow cooker. Place the seared rib roast on top and cover. Cook on low for 3-3.5 hours or until internal temperature reaches 130-134°F for medium-rare.

Remove the prime rib and let it rest for 20-30 minutes covered loosely with foil.

Strain the cooking liquid in a saucepan, add Worcestershire sauce, and xanthan gum. Simmer until thickened. Taste and season as needed.

Slice the rested prime rib thinly against the grain, arrange on a platter and ladle gravy over the beef before serving.