In a large mixing bowl, combine the ground chicken, minced garlic, chopped green onions, and fresh spinach. Add the crumbled feta cheese along with your chosen herbs, salt, and pepper. Mix gently with your hands until well incorporated.

Divide the mixture into equal portions and shape each one into a burger patty, roughly 3/4-inch thick. Press the centers slightly to prevent puffing during cooking.

Fire up your grill to medium-high heat, allowing it to preheat for about 10 minutes. Ensure the grill grates are clean and lightly greased.

Carefully place the patties on the grill and cook for 4-5 minutes on one side. Once they develop a golden-brown crust, flip them and grill for another 4-5 minutes. Use a meat thermometer to check that the internal temperature reaches 165°F (75°C).

Prepare the tzatziki sauce by combining Greek yogurt, grated cucumber, minced garlic, fresh lemon juice, and dill in a separate bowl. Stir until well blended, and season with salt to taste.

Once the burgers are cooked, remove them from the grill and let them rest for a couple of minutes. Place each patty on a bun or lettuce, then generously top with the tzatziki sauce.