1. MAKE THE TURKEY BURGERS: In a small skillet, heat the olive oil over medium heat. Add the onion and cook until tender, 3 to 4 minutes. Add the garlic and sauté until fragrant, 1 minute more. Set aside to cool to room temperature.

2. In a medium bowl, mix the cooled onion mixture with the egg, parsley, oregano, red-pepper flakes and ground turkey. Add the bread crumbs, season with salt and pepper, and mix until combined.

3. Preheat the oven to 375°F. Form the meat mixture into four patties of equal size. Heat a large oven-safe skillet over medium-high heat and spray it generously with nonstick cooking spray.

4. Place the burgers in the skillet and sear on each side until well browned, 4 to 5 minutes per side. Transfer the skillet to the oven and cook until the burgers are fully cooked through, 15 to 17 minutes more.

5. MAKE THE TZATZIKI SAUCE: In a medium bowl, mix the yogurt with the cucumber, olive oil, lemon juice and garlic powder to combine. Season with salt and pepper, and then stir in the parsley.

6. MAKE THE TOPPINGS: Place each burger on the bottom half of a bun and then top each with about ¼ cup tzatziki, two lettuce leaves, two tomato slices and the top half of the bun. Serve immediately.