01 - Combine ground turkey, breadcrumbs, egg, red onion, garlic, Worcestershire sauce, salt, and pepper in a large bowl. Mix gently until just combined; avoid overmixing to prevent tough burgers.
02 - Divide the mixture into 4 equal portions and shape into patties approximately 1/2 inch thick. Make a slight indentation in the center of each patty to prevent puffing during cooking.
03 - Preheat a grill or skillet over medium-high heat. Lightly oil the surface to prevent sticking.
04 - Cook the patties for 5–6 minutes per side until fully cooked, reaching an internal temperature of 165°F.
05 - Place a slice of Swiss cheese on each patty during the last minute of cooking. Cover the grill or skillet to melt the cheese completely.
06 - Toast the burger buns cut side down until golden and lightly crispy.
07 - Spread mayonnaise and/or mustard on the toasted buns if desired.
08 - Layer each burger with lettuce, turkey patty with melted cheese, avocado slices, and tomato. Top with the remaining bun halves.
09 - Serve the burgers immediately while hot and the cheese is still melted.