Add the marinade ingredients to a large Ziploc bag, add the chicken and massage the marinade into the meat. Allow to marinate for at least 1 hour, or preferably overnight.
Add sliced onions to a medium bowl. Pour enough boiling water over them to fully cover and count to 10. Drain onions and place in jar or bowl.
Heat vinegar and sugar in a small saucepan over medium heat until sugar has dissolved, about 3 minutes.
Pour vinegar mixture over onions. Allow to stand at room temperature, until fully cooled, about 20 minutes.
Cut the cucumber in half lengthwise and scoop out the seeds. Coarsely grate the cucumber using a box grater or food processor. Then wrap in paper towels or a tea towel and squeeze to remove excess liquid.
Place cucumber in a bowl. Add remaining ingredients then mix to combine. Set aside for at least 20 minutes for the flavors combine.
Remove chicken from marinade and cook in a large skillet or grill pan over medium heat 5-6 minutes on each side until cooked through. Cut into strips.
Divide quinoa between bowls. Top with chicken strips, pickled onion, tomato, cucumber and feta. Drizzle with tzatziki sauce and garnish with parsley and mint.