Place the beef broth and dry onion soup mix into a large crock pot, stir until combined. Season the roast beef with the pepper and the garlic. Place the oil in a large skillet over medium-high heat, then sear the roast on all sides, then place the beef roast into the liquid in the slow cooker.
Cover and cook on low for 6-8 hours or high for 3-4 hours until the meat is fork-tender. Remove the roast from the crock pot and slice into thin slices or shred. If you are shredding the beef it is best to leave some liquid with it to ensure it stays moist and delicious!
Place the mushrooms into a medium sized skillet and add in the butter. Cook the mushrooms until they are tender and have shrunk in size, and are golden brown.
Preheat your oven to 350 °. Place some beef into the bun, top with some cooked mushrooms and then the cheese slices and place on a baking sheet.. Toast the sandwiches for 5 minutes or until the cheese starts to melt.
Strain the liquid in the slow cooker though a sieve into a bowl, then ladle the liquid (au jus) into bowls large enough for dipping for when you are serving.
If desired, add some mushrooms into the gravy!
Instant Pot Instructions
Press the saute button and place the oil into the bottom pot of the Instant Pot. Season the roast beef with the pepper and the garlic..Sear the roast on all sides, then turn off the saute button.
Add the broth and the onion soup mix to the pot. Place the lid on to of the Instant Pot and seal according to manufacturers directions. Cook the roast on high pressure/manual for 60 minutes.
Let the pot naturally release for 10 minutes, then manually release according to manufacturers directions.
Remove the roast from the pressure cooker and slice into thin slices or shred. If you are shredding the beef it is best to leave some liquid with it to ensure it stays moist and delicious!
Place the mushrooms into a medium sized skillet and add in the butter. Cook the mushrooms until they are tender and have shrunk in size, and are golden brown.
Preheat your oven to 350 °. Place some beef into the bun, top with some cooked mushrooms and then the cheese slices and place on a baking sheet.. Toast the sandwiches for 5 minutes or until the cheese starts to melt.
Strain the liquid in the slow cooker though a sieve into a bowl, then ladle the liquid (au jus) into bowls large enough for dipping for when you are serving.
Notes
There is enough beef for more than 4 sandwiches, this will make a good 8!
If you want shredded beef, you can cook the roast ALL DAY in the slow cooker, or for 90 minutes in the Instant Pot. The times shown are for meat that can be sliced!