Marinate the chicken: Combine olive oil, lemon juice, oregano, garlic powder, paprika, salt, and pepper. Coat chicken and marinate for at least 15 minutes.
Make the tzatziki: Grate cucumber, squeeze out excess water in a clean towel, then mix with Greek yogurt, garlic, lemon juice, dill, salt, and pepper. Refrigerate until serving.
Cook the chicken: Heat a grill pan over medium-high heat. Cook chicken 6-7 minutes per side until internal temperature reaches 165°F.
Rest and slice: Let chicken rest 5 minutes, then slice into strips or bite-sized pieces.
Prepare components: Cook quinoa or rice according to package directions. Prep all vegetables while chicken cooks.
Assemble bowls: Divide grain between bowls, top with chicken, vegetables, feta, and olives. Drizzle generously with tzatziki and serve with lemon wedges.