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 maintain tenderness.
02 - Divide mixture into 4 equal portions and shape into patties approximately 1/2 inch thick. Make a slight indent in the center of each patty to prevent puffing during cooking.
03 - Preheat grill or skillet over medium-high heat. Lightly oil the surface to prevent sticking.
04 - Place patties on the preheated surface and cook for 5-6 minutes per side. Cook until fully cooked through, reaching an internal temperature of 165°F.
05 - During the final minute of cooking, place a slice of Swiss cheese on each patty. Cover with a lid to melt the cheese completely.
06 - Toast the burger buns cut side down until golden and lightly crisped.
07 - Spread mayonnaise and/or Dijon mustard on the toasted buns if desired.
08 - Layer each burger starting with lettuce on the bottom bun, followed by the turkey patty with melted cheese, avocado slices, and tomato. Top with the remaining bun half and serve immediately.