Place the pita bread in a toaster, toaster oven or oven and toast for a few minutes until just slightly toasty. Remove and set aside.
Combine the cooked chicken, tzatziki sauce and red onion in a large bowl and toss to combine. Taste and season with salt and pepper as needed.
Divide the mixture into 4 equal parts and stuff each into a pita pocket and top with avocado slices.