Preheat your oven to 325 F.
In a bowl, combine the garlic, onion, oregano, and salt and pepper to taste.
2 tbsp. onion flakes, 1 tbsp. garlic flakes, 1 tbsp. dried oregano, Sea salt and freshly ground black pepper
Rub the seasoning blend all over the roast.
2 ½ lbs. beef rib roast
Place the roast on a rack in a roasting pan.
Roast in the preheated oven, uncovered, for about 2 hours or to your desired doneness.
Take the roast out of the pan, but don’t rinse out the pan. Set the roast aside to rest for about 10 minutes.
While the meat is resting, heat the roasting pan on the stovetop over medium heat. Pour in the beef stock, scraping up the bottom of the pan.
2 cups beef stock
Add in the vinegar and bring to a boil. Simmer until slightly reduced, and season to taste.
¼ cup balsamic vinegar
Add the mushrooms and cook until it starts to soften.
2 portobello mushrooms
Serve the rib roast with the balsamic-portobello sauce.