01 - In a large bowl, combine ground turkey, breadcrumbs, egg, red onion, garlic, Dijon mustard, salt, and black pepper. Mix gently until just combined.
02 - Form the mixture into 4 equal patties, pressing a slight indentation in the center of each for even cooking.
03 - Heat olive oil in a grill pan or skillet over medium heat. Cook patties for 5-6 minutes per side, or until cooked through and golden brown (internal temperature should reach 165°F).
04 - In the last minute of cooking, top each patty with a slice of Swiss cheese and cover the pan to melt the cheese.
05 - Toast the burger buns until golden.
06 - To assemble, spread mayonnaise and/or ketchup on the bottom bun (if using). Layer with lettuce, turkey patty with melted cheese, avocado slices, tomato, and red onion. Top with the other half of the bun.
07 - Serve immediately with your favorite sides.