Chicken Gyros with Tomato Tzatziki

Sauce

• 1 cup plain Greek yogurt
• 2 tablespoons finely chopped fresh mint leaves
• 1 tablespoon extra-virgin olive oil
• 1½ teaspoon fresh lemon juice
• ½ teaspoon kosher salt
• ¼ teaspoon freshly ground black pepper
• 1 cup seeded and finely chopped tomato
• ½ cup finely diced English cucumber

Marinade

• 3 tablespoons extra-virgin olive oil
• 3 tablespoons fresh lemon juice
• 1 tablespoon dried oregano
• 1 garlic clove, minced
• ½ teaspoon kosher salt
• ¼ teaspoon freshly ground black pepper
• ⅛ teaspoon crushed red pepper flakes

• 1½ pound chicken breast tenders
• 1 small red onion, cut crosswise into ½-inch slices
• Extra-virgin olive oil
• 6 soft flatbreads or pitas