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 mixture into 4 equal portions and shape into burger patties approximately 3/4-inch thick.
03 - Preheat a grill or skillet over medium-high heat. Lightly oil the grates or pan surface.
04 - Cook 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. Cover briefly to allow cheese to melt completely.
06 - Toast burger buns until golden brown.
07 - Spread mayonnaise on bottom buns if desired. Layer with arugula or lettuce, followed by turkey patty with melted cheese, avocado slices, and red onion. Crown with top buns.
08 - Serve immediately with preferred side dishes.