If your chicken breasts are thick, pound them to an even 1/2-inch thickness. In a medium bowl, whisk together olive oil, lemon juice, minced garlic, oregano, paprika, salt, and pepper. Add chicken pieces and turn to coat all sides. Cover and refrigerate for 15 minutes, or up to 2 hours for deeper flavor.

Heat a grill pan or large skillet over medium-high heat for 2-3 minutes until a water droplet sizzles and evaporates on contact. Add marinated chicken and cook for 6 to 7 minutes per side. The chicken is done when it reaches an internal temperature of 165°F and the juices run clear. Remove from heat and let rest for 5 minutes before slicing into strips or bite-sized cubes.

While chicken cooks, prepare your tzatziki. Halve the cucumber lengthwise and scoop out the seeds with a spoon. Grate using the large holes of a box grater, then wrap the grated cucumber in a clean kitchen towel and twist tightly over the sink until no more liquid drips out (you should extract 2-3 tablespoons of liquid). Combine Greek yogurt, squeezed cucumber, lemon juice, olive oil, minced garlic, and dill in a bowl. Season generously with salt and pepper, then stir until smooth and thick enough to coat the back of a spoon.

In a large serving bowl, combine chopped greens, halved cherry tomatoes, sliced red onion, cucumber rounds, olives, and feta cheese if using. Top with your sliced chicken.

Drizzle tzatziki dressing over the entire salad and toss gently to combine, making sure the dressing coats everything. Serve immediately for a fresh crunch, or chill for 10-15 minutes if you prefer it cold.