In a bowl or plastic bag, combine the chicken thighs with olive oil, minced garlic, apple cider vinegar, thyme, oregano, parsley, sea salt, and pepper to create the marinade and soak the chicken for at least 30 minutes or preferably overnight.
To prepare the pickled onions, heat apple cider vinegar, water, salt, and date syrup in a saucepan until nearly boiling, then pour the hot liquid over the thinly sliced red onions in a heat-safe container, pressing the onions down with a fork to cover them completely; let them rest at room temperature for at least 30 minutes and then refrigerate.
Mix all ingredients for the dairy free Tzatziki sauce thoroughly; it can be made up to two days in advance and refrigerated for up to five days.
Heat the oven to 400°F (204°C), line a baking sheet with parchment or foil, arrange the marinated chicken thighs on it, discard leftover marinade, then bake for 25 to 30 minutes until the chicken reaches an internal temperature of 165°F and the edges are crisp; let the chicken rest for 5 minutes before dicing.
Assemble the bowls by placing a bed of spinach or lettuce, top with sliced tomatoes, diced cucumber, black olives, pickled onions, diced chicken, and a generous spoonful of the Tzatziki sauce; serve and enjoy immediately.