In a large bowl, combine olive oil, minced garlic, dried oregano, ground cumin, smoked paprika, salt, pepper, and lemon juice. Add the chicken thighs and coat them evenly. Cover and marinate in the refrigerator for at least 30 minutes (or up to overnight for better flavor).
Preheat a grill or grill pan over medium-high heat. Remove chicken from the marinade and grill for about 6-7 minutes per side, or until the internal temperature reaches 165°F (75°C). Remove from heat and let it rest for a few minutes before slicing.
While the chicken rests, prepare the quinoa according to package instructions if not pre-cooked. Divide the cooked quinoa equally among serving bowls.
Top the quinoa with sliced grilled chicken, halved cherry tomatoes, diced cucumber, thinly sliced red onion, and shredded lettuce.
Sprinkle crumbled feta cheese over each bowl and drizzle with tzatziki sauce. If desired, warm pita bread in a toaster or on the grill.
Garnish the bowls with extra lemon wedges and fresh herbs if desired.
Notes
Marinating the chicken overnight enhances the flavor.