Place the chicken and the onions in a large freezer bag or plastic container; set aside.
Place the olive oil, lemon juice, garlic, and all of the spices into a large bowl and whisk until the mixture is well blended. Carefully pour the mixture into the bag or container, over the chicken. Seal the bag or container, then mix and shake until the chicken and onions are well coated.
Transfer the chicken and onions into your Instant Pot and mix things around to ensure even coverage. Close the lid (follow the manufacturer's guide for instructions if needed). Set the Instant Pot to the Manual setting, and set the timer to 8 minutes on high pressure. In the meantime, prep your tomatoes and cucumbers.
When the Instant Pot cycle is done, wait until the natural release cycle is complete—this should take about 15 minutes. (If you're in a rush, check the manufacturer's manual for quick release.) Carefully unlock and remove the lid from the Instant Pot. Let the steam release itself. When done and when safe, take two forks and shred the chicken so it resembles pulled chicken. Stir the chicken around to coat and let it sit for a few minutes to soak up the sauce.
Serve with lettuce cups, tomatoes, cucumbers, and tzatziki or hummus.