01 - Combine ground turkey, breadcrumbs, egg, parsley, Dijon mustard, minced garlic, onion powder, salt, and black pepper in a large bowl. Mix until just combined without overworking the meat.
02 - Divide the 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 the patties for 5 to 6 minutes per side until fully cooked, reaching an internal temperature of 165°F.
05 - During the last minute of cooking, place one slice of Swiss cheese on each patty and cover briefly to allow melting.
06 - Toast the burger buns until golden brown.
07 - Spread mayonnaise on the bottom buns if using. Layer with arugula or lettuce, turkey patty with melted cheese, avocado slices, and red onion. Top with remaining buns.
08 - Serve immediately with preferred accompaniments.