Cut the onion into wedges and peel the garlic. Generously season the piece of meat on both sides with coarse salt and pepper.
Place the stainless steel inner pot inside the Instant Pot®. Press Saute, wait a couple of minutes so it heats up, then add a splash of olive oil and sear the meat on all sides (approx. 1 minute on each side). Remove and set aside.
Turn off the Instant Pot® and add 1 cup beef broth. With a wooden spoon, carefully scrape the drippings from the bottom and add the onion segments and 3 garlic cloves. Then add the seared meat and 1 more cup beef broth, soy, Worcestershire sauce and thyme.
Close and lock the Instant Pot® lid. Make sure the steam release handle is turned to Sealing. Press Pressure Cook, pressure level High, and set the time to 45 Minutes.
Once the cooking time is up, let the pressure release naturally At Least10 minutes (although 15-20 minutes is best) before manually releasing pressure.
Open the lid and remove the meat, then shred using two forks. Meanwhile, press Saute for 15 Minutes (with the Instant Pot® open) to reduce the cooking broth.
Open the bread, butter on both sides and heat in the preheated oven for 5 minutes at 355°F (180°C). Remove from the oven; rub a clove of garlic on both sides, place a layer of shredded meat and top with a slice of mantecoso cheese and put back in the oven for 5 minutes or until the cheese melts.
Pour some broth into a bowl and serve with the sandwich. Dip your sandwich in the broth. It will be unbelievably tasty!