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.
Pickled Onions:
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.
Tzatziki:
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.
Chicken:
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.
Assembly:
Divide quinoa between bowls. Top with chicken strips, pickled onion, tomato, cucumber and feta. Drizzle with tzatziki sauce and garnish with parsley and mint.