In a food processor, add your onion and process until it's like a paste. It will be very wet.
In a medium bowl, add the ground turkey. Using your hands, mix in the onion, panko, dried oregano, garlic powder, dill seeds and salt. Divide the meat into four portions and shape into patties. Place in the refrigerator to chill for at least 1 hour.
Remove burgers from the refrigerator and cook in skillet over medium high heat for 5 to 6 minutes on each side.
Place cooked burger into a pita crammed with baby spinach, cucumber, red onion, crumbled feta and tzatziki.
Note: I recommend not skipping refrigerating the burgers since the processed onion make them very wet. The refrigerator helps firm them up a bit so they're easier to cook.