Slice the onion into ~1/4-inch rounds. Keeping the rounds intact, place them in the bottom of the crockpot.
Sprinkled the meat with salt, pepper and onion powder. Heat a large saute pan over medium-high heat and add 2 tsp oil/butter. Sear the meat on all sides until golden brown to seal in the juices and give the meat a little color.
Add beef broth, soy sauce/tamari/ aminos, mustard, rosemary and garlic to the crockpot.
Place meat on top of the onions.
Cook on low 6-7 hours until beef is fall-apart tender.
Shred meat with a fork or transfer the roast to a cutting board and thinly slice.
Remove the onions and set aside. I strain the juice into a large “defatter” measuring cup and get rid of the oil that collects on top (I .
Return the roast, juice, and onions to the crockpot for up to an hour before serving (I usually switch it to “Keep Warm” so it doesn’t continue cooking).
While meat is on "warm", heat a medium saute pan with remaining 1 tsp of oil/butter. Add mushrooms and cook until golden brown, stirring often. Season with salt and pepper is desired.
Heat the broiler. Place the split sandwich rolls on a lined baking sheet and toast for 1 minute or just until the bread begins to brown.
Remove the tops from the pan. Lay the beef on the bottom of the rolls and top with mushrooms and cheese.
Return to the oven just until cheese is melted. Top with onions and sandwich halves and serve with small bowls of au jus, Enjoy!
