For the Tzatziki Sauce: Combine all ingredients and stir well. Refrigerate until ready to serve.

Remove any large pieces of fat from the beef roast and then slice the meat into thin, 1/4 inch strips. Place in the bottom of the slow cooker.

In a small bowl combine the olive oil, garlic powder, oregano, cumin, thyme, lemon juice, salt, and pepper. Pour over meat. Cover and cook on LOW for 6 hours or until beef is tender.

Add a lettuce leaf to a pita. Top with meat, tomatoes, cucumber, onion and tzatziki sauce.

Remove any large pieces of fat from the beef roast and then slice the meat into thin, 1/4 inch strips. Place in Instant pot.

In a small bowl combine 1/3 cup of WATER, olive oil, garlic powder, oregano, cumin, thyme, lemon juice, and salt and pepper. Pour over meat.

Cover and turn the valve to "sealing." Press "manual" or high pressure, and set the timer for 30 minutes.

After the timer beeps allow the instant pot to rest for 15 minutes before releasing any of the pressure. Remove the lid, taste, and add additional seasonings if needed.

Add a lettuce leaf to a pita. Top with meat, tomatoes, cucumber, onion and tzatziki sauce.