To marinate chicken:
Cut chicken in half crosswise. Cover with plastic wrap and use a kitchen mallet or the bottom of a glass to pound to about 1 inch thick.
Combine olive oil, lemon juice, garlic, salt and spices in a large ziplock bag. Add chicken and seal. Use your hands to work the marinade all over the chicken.
Chill in the fridge for at least 4 hours and up to 24 hours.
To make the tzatziki sauce:
Combine the yogurt, lemon juice, garlic, cucumber, mint, salt and pepper in a bowl.
Chill in the fridge for at least 1 hour and up to several days.
To make sandwiches:
Preheat grill to medium-high heat, or preheat heavy bottom skillet (I like cast iron) over medium-high heat on stove.
Cook chicken on both sides for 4-5 minutes, until browned and cooked all the way through. An instant-read thermometer should read 165°F.
If desired, brush buns with olive oil and toast on grill or skillet until lightly browned, about 2 minutes.
Assemble sandwiches with buns, chicken, tzatziki sauce, lettuce, tomato, and feta cheese. Serve immediately.
Calories: 431kcal, Carbohydrates: 26g, Protein: 36g, Fat: 19g, Saturated Fat: 5g, Cholesterol: 92mg, Sodium: 1448mg, Potassium: 584mg, Fiber: 1g, Sugar: 5g, Vitamin A: 368IU, Vitamin C: 5mg, Calcium: 236mg, Iron: 2mg