01 - In a large bowl, gently mix ground turkey, red onion, parsley, garlic, Dijon mustard, Worcestershire sauce, salt, and black pepper until just combined.
02 - Divide the mixture into four equal portions and form 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 patties for 5 to 6 minutes per side until golden brown and internal temperature reaches 165°F.
05 - Place Swiss cheese slices on patties during the last minute of cooking, cover pan to allow cheese to melt.
06 - Toast the burger buns until golden brown.
07 - Spread mayonnaise and/or Dijon mustard on the bottom halves of buns if desired. Layer with lettuce, turkey patty topped with Swiss cheese, tomato slices, avocado slices, and cover with the top bun.
08 - Serve immediately while warm.