In a bowl or zip-top bag, combine olive oil, lemon zest and juice, garlic, oregano, salt, pepper, and red pepper flakes. Add chicken, toss to coat, and marinate for at least 15–30 minutes (longer is even better).
Combine all tzatziki ingredients in a bowl. Stir well and refrigerate until ready to use.
Heat a grill pan or skillet over medium-high heat. Cook chicken for 4–5 minutes per side, or until golden and cooked through (internal temp 165°F). Let rest 5 minutes, then slice.
Divide rice (or grain of choice) among 4 bowls. Top with chicken, tomatoes, cucumber, olives, red onion, and feta. Add a generous dollop of tzatziki. Squeeze fresh lemon over the top and serve with pita wedges if you like.