In a large bowl, combine the ground turkey with the green onion, breadcrumbs, milk, egg, chili flakes, salt, pepper, and crumbled feta cheese. Thoroughly, mix to combine and use your hands to shape into 6 patties.
Pre-heat grill to medium heat or indoor electric grill to high heat and grill for approximately 5-6 minutes on each side, or until internal temperature registers 165 degrees F.
As the turkey burgers cook, prepare the toppings. For the yogurt sauce, combine the Greek yogurt with the cucumber, red onion, dill, and salt and pepper, to taste.
Just before serving, lightly toast your hamburger buns and assemble. Start with the lettuce on the bottom, followed by the turkey burger, roasted bell pepper, tomato, and cucumber yogurt sauce. Enjoy!