01 - In a large mixing bowl, combine ground turkey, breadcrumbs, egg, parsley, Dijon mustard, minced garlic, onion powder, salt, and black pepper. Mix until just combined without overworking the mixture.
02 - Divide the mixture into 4 equal portions and shape each into a burger patty approximately 3/4 inch thick.
03 - Preheat a grill or skillet over medium-high heat and lightly oil the grates or cooking surface.
04 - Cook the patties for 5 to 6 minutes per side until fully cooked, reaching an internal temperature of 165°F.
05 - During the final minute of cooking, place one slice of Swiss cheese on each patty and cover briefly to allow the cheese to melt.
06 - Toast the burger buns until golden brown.
07 - Spread mayonnaise on the bottom buns if desired. Layer with arugula or lettuce, topped with the turkey patty and melted cheese, avocado slices, and red onion. Place the top bun on each burger.
08 - Serve the burgers immediately with your preferred side dishes.