01 - In a spacious 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.
02 - Divide the mixture into four equal portions. Shape each portion into a patty, pressing a slight indentation in the center of each to ensure uniform cooking.
03 - Heat the olive oil in a grill pan or skillet over medium heat. Cook the patties for 5-6 minutes per side, or until thoroughly cooked through and golden brown (an internal temperature of 165°F is recommended).
04 - During the final minute of cooking, place one slice of Swiss cheese atop each patty. Cover the pan briefly to facilitate the melting of the cheese.
05 - Lightly toast the split burger buns until they achieve a golden hue.
06 - To construct the burgers, spread mayonnaise and/or ketchup on the bottom bun, if desired. Layer with lettuce, the turkey patty crowned with melted cheese, avocado slices, tomato, and thinly sliced red onion. Place the top half of the bun to complete the burger.
07 - Serve the turkey burgers immediately with your preferred side dishes.