01 - Combine ground turkey, breadcrumbs, egg, red onion, garlic, Worcestershire sauce, salt, and pepper in a large bowl. Mix gently until just incorporated, taking care not to overwork the meat.
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 grill or skillet over medium-high heat. Lightly oil the cooking 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 Swiss cheese slice on each patty. Cover the grill or skillet to allow the cheese to melt completely.
06 - Toast the split burger buns cut side down until golden brown and lightly crisped.
07 - Spread mayonnaise and/or Dijon mustard on the toasted bun halves, if desired.
08 - Layer each burger by placing lettuce on the bottom bun, followed by the cheesy turkey patty, avocado slices, and tomato. Top with the remaining bun half.
09 - Serve the assembled burgers immediately while the patties are hot and the cheese is melted.