01 - In a large bowl, gently mix ground turkey, chopped red onion, parsley, minced 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 - Warm olive oil in a 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 (74°C).
05 - In the last minute of cooking, place one slice of Swiss cheese atop each patty; cover pan to allow cheese to melt.
06 - Toast burger buns until golden brown.
07 - Spread mayonnaise and/or Dijon mustard on the bottom half of each bun if desired. Layer lettuce, turkey patty with melted cheese, tomato slices, avocado slices, and top bun.
08 - Serve immediately while warm.