Marinate the chicken: In a bowl, mix olive oil, lemon zest and juice, garlic, oregano, cumin (if using), salt, and pepper. Add chicken and toss to coat.
Let it marinate for at least 20–30 minutes, or up to 8 hours in the fridge.
Cook your base: Prepare rice or quinoa according to package directions. Fluff and keep warm, or chill for meal-prep bowls.
Make the tzatziki: Grate the cucumber, then squeeze out excess water with a clean towel. Stir together yogurt, cucumber, garlic, lemon juice, olive oil, dill, salt, and pepper.
Taste and adjust salt or lemon as needed. Chill until serving.
Grill or pan-sear the chicken: Heat a grill or skillet over medium-high. Cook chicken 5–7 minutes per side, depending on thickness, until the internal temperature reaches 165°F (74°C).
Rest 5 minutes, then slice.
Prep the veggies: Halve tomatoes, dice cucumber, and slice red onion. Pit and halve olives if using. Crumble feta.
Assemble the bowls: Add a scoop of rice or quinoa to each bowl.
Top with sliced chicken, tomatoes, cucumber, red onion, olives, and feta.
Finish with tzatziki: Spoon a generous dollop over the top. Sprinkle with herbs and serve with lemon wedges. Add a drizzle of olive oil if you like.