Pound chicken breasts to an even thickness of about 1/2 inch. Place chicken in a zip close bag with olive oil and greek seasoning. Smoosh around to evenly coat with seasoning and olive oil and let marinate for about 10-15 minutes.
Heat grill or George Foreman grill, and grill chicken until internal temp reaches 165 degrees, approximately 5 to 6 minutes per side.
Remove from grill, place on plate then loosely cover with foil and let rest for 5 minutes before cutting. Slice chicken into strips.
Construct pita sandwich as you desire, with spinach leaves, tzatziki sauce, feta cheese, tomato slices, cucumbers and purple onion. Enjoy!