Season and sear. Season the pot roast with salt, pepper, and garlic salt. Set the Instant Pot to saute and heat the olive oil in the Instant Pot. Add the pot roast. Sear for 2 minutes on either side. Transfer the meat to a plate and set aside.
Load the Instant Pot. Add the beef broth, onion, and garlic to the Instant Pot. Set the seared pot roast on top of the onions. Drizzle the Worcestershire sauce and soy sauce over the meat and season with thyme and rosemary.
Cook. Seal the Instant Pot and cook on high pressure for 40 minutes followed by a natural release.
Rest. Transfer the meat to a cutting board and allow it to rest for 10 minutes. You can discard the remaining contents of the Instant Pot.
Slice. Thinly slice or shred the pot roast and set aside.
Topping
Caramelize the onions. Melt the butter over medium heat in a large skillet. Add the onions and saute until caramelized. If the pan starts to dry up, add a splash of water or beef broth.
Saute the mushrooms. Add the mushrooms to the skillet, season with salt and pepper, and saute until golden brown. Turn off the heat and set aside.
Chipotle Aioli
Whisk. Whisk together the mayo, adobo sauce, and garlic powder
Season. Taste and season with salt and pepper if desired.
Sandwiches
Assemble. Brush the outside of each slice of bread with mayo. Brush the inside with aioli. Arrange ¼ of the sliced or shredded pot roast over a slice of bread (aioli side) and top with ¼ of the mushroom mixture and 2 slices of cheese. Close the sandwich with another slice of bread. Repeat with the remaining ingredients to make 3 more sandwiches.
Melt. Heat a large heavy bottom skillet over medium heat. Add the butter and let it melt. Add the sandwiches (you may need to do this in batches) and cover the pan. Cook for 3 minutes. Flip, replace the lid, and cook for an additional 3 minutes or until the cheese has melted.
Notes
You can pour some of the leftover juice from the beef into a small bowl and use it for dipping once you have your prepared sandwich.