Season the beef on both sides with the salt and pepper. Put the beef and onion into the Instant Pot.
In a small bowl, whisk together the water, oil, lemon juice, oregano, and garlic powder, then add to the pot.
Lock the lid in place and turn the valve to Sealing. Press the Manual/Pressure Cook button. Set the cook time for 30 minutes at high pressure.
While the beef cooks, make the tzatziki: In a small bowl, whisk together yogurt, cucumber, garlic, dill, and lemon juice. Season to taste with salt and pepper. Set aside until ready to serve.
Let the steam release naturally for about 15 minutes, then turn the valve to Venting to release any residual steam. Remove the lid from the pot and, using a slotted spoon, transfer the beef and onion to a bowl.
To serve, spoon an equal amount of the beef mixture into each pita and top with lettuce, tomato, and onion. Sprinkle with chopped dill and finish with a dollop of tzatziki.