Season the chicken: In a large bowl, whisk together the olive oil, garlic, oregano, paprika, lemon juice, salt, and black pepper. Add the chicken thighs and toss until evenly coated. Let the chicken sit while you prepare the rest of the ingredients, or cover and refrigerate for up to 24 hours.
Make the lemon dressing: In a small bowl or jar, combine the olive oil, lemon juice, red wine vinegar, garlic, oregano, salt, and black pepper. Whisk or shake until well combined.
Prepare the bowl ingredients: Cook the quinoa if needed. Chop the cucumber, halve the cherry tomatoes, thinly slice the red onion, crumble the feta, and gather the arugula, Kalamata olives, tzatziki, and lemon wedges.
Cook the chicken: Heat a large skillet over medium heat. Add the chicken thighs and cook for 5-7 minutes per side, or until golden brown and the internal temperature reaches 165°F. Transfer the chicken to a cutting board and let it rest for 5 minutes before slicing.
Assemble the bowls: Divide the cooked quinoa and arugula between four bowls. Top with sliced chicken, cherry tomatoes, cucumber, red onion, Kalamata olives, and feta cheese.
Finish and serve: Drizzle each bowl with lemon dressing, add a generous spoonful of tzatziki, and serve with extra lemon wedges on the side.