01 - In a large mixing bowl, combine the ground turkey, breadcrumbs, egg, finely chopped red onion, minced garlic, Dijon mustard, salt, and black pepper. Mix gently until all ingredients are just incorporated, being careful not to overmix.
02 - Divide the mixture into 4 equal portions and form them into patties. Press a slight indentation into the center of each patty; this technique helps prevent them from bulging during the cooking process.
03 - Heat 1 tablespoon of olive oil in a grill pan or skillet over medium heat. Place the prepared patties in the pan and cook for 5 to 6 minutes per side, or until the internal temperature reaches 165°F and they achieve a golden-brown exterior.
04 - During the final minute of cooking, top each turkey patty with a slice of Swiss cheese. Cover the pan briefly to allow the cheese to melt perfectly over the hot patties, ensuring a creamy texture.
05 - Toast the whole wheat burger buns until they are lightly golden brown and warmed through, providing a sturdy base for the burger.
06 - To assemble each burger, spread mayonnaise and/or ketchup on the bottom half of a toasted bun, if desired. Layer with a fresh lettuce leaf, a turkey patty topped with melted Swiss cheese, creamy avocado slices, a slice of ripe tomato, and thin slices of red onion. Place the top half of the bun over the assembled toppings.
07 - Serve the turkey burgers immediately upon assembly with your preferred side dishes for the best culinary experience.