First, combine all of the chicken marinade ingredients. Place the chicken thighs in a shallow dish or a plastic bag and pour the marinade over top. Let them marinate for a minimum of 30 minutes (or overnight).
Make the pickled onions. Combine the apple cider vinegar, water, salt, and date syrup in a small sauce pan and bring it almost to a boil. Place your thinly-sliced onion in a glass bowl/container (or another heat-resistant container) and pour the mixture over top. Use a fork to press the sliced onions down into the mixture to coat them. Let them sit on the counter for 30 minutes or more. You can store these in the fridge for 1-2 weeks!
Make the Tzatziki sauce. Combine all of the ingredients and stir well. You can make this up to 2 days ahead of time and store in the fridge for up to 5 days.
Preheat the oven to 400. Line a baking sheet with parchment paper or foil. Place the chicken thighs on the baking sheet and discard any remaining marinade. Bake them for 25-30 minutes or until their internal temp reaches 165 and they are crisped to your liking. Let them rest for 5 minutes after they come out of the oven, then dice them up.
Assemble your bowls! Add some spinach or lettuce, tomatoes, cucumber, olives, pickled onions, chicken, and tzatziki sauce to each bowl. Enjoy!