Pour beef broth into the bottom of the Instant Pot.
Place beef roast in the Instant Pot. Top with sliced onions.
In a small bowl, mix together olive oil, garlic, oregano, lemon juice, salt, and pepper.
Pour olive oil mixture over the roast.
Place lid on pot and press MANUAL (or PRESSURE COOK) and set timer for 60 minutes (you want to cook beef roasts at a high pressure for 20 minutes per pound).
Allow 10-15 minutes to come to pressure before it starts the countdown cook timer.
Once it is finished cooking, do a natural release of the pressure for at least 10 minutes (20 minutes is ideal). Remove the lid.
Slice or shred the beef and serve on pitas with cucumbers, tomatoes, and homemade tzatziki sauce.