Marinate the chicken in olive oil, lemon juice, garlic, Greek yogurt, oregano, salt, and pepper for at least 20 minutes (overnight for best flavor).
Cook 1 cup quinoa in 1 3/4 cups 2 water until tender (~12minutes). Fluff and cool.
Grate the cucumber for the tzatziki and squeeze dry. Combine with Greek yogurt, garlic, lemon, dill, and salt. Refrigerate.
Toss cooled quinoa with cucumber, tomatoes, red onion, olives, feta. Whisk dressing ingredients and pour over; toss to coat.
Grill chicken thighs at medium-high heat for 5–7 minutes per side OR air fry at 390°F for 16 minutes flipping halfway, until 165°F internal.
Rest chicken 5 minutes, then slice.
Build bowls: quinoa salad as the base, sliced chicken on top, dollop of tzatziki, pita on the side.