01 - Combine ground turkey, red onion, parsley, garlic, Dijon mustard, Worcestershire sauce, salt, and black pepper in a large bowl. Mix gently until just combined.
02 - Divide mixture into 4 equal portions and shape each into a patty approximately 3/4 inch thick.
03 - Heat olive oil in a large skillet or grill pan over medium heat.
04 - Cook turkey patties for 5 to 6 minutes per side until golden brown and internal temperature reaches 165°F (74°C).
05 - During the final minute of cooking, place a slice of Swiss cheese on each patty and cover the pan to melt the cheese.
06 - Toast the whole wheat buns until golden and warm.
07 - Spread mayonnaise and/or Dijon mustard on the bottom halves of the buns if desired. Layer with lettuce, turkey patty with melted Swiss cheese, tomato slices, avocado slices, and top with bun halves.
08 - Serve immediately while warm.