01 - In a large bowl, mix ground turkey, finely chopped onion, minced garlic, Worcestershire sauce, Dijon mustard, salt, and black pepper until just combined.
02 - Divide the mixture into 4 equal portions and form each into a patty approximately ¾ inch thick.
03 - Heat olive oil in a skillet or grill pan over medium heat. Cook patties for 5 to 6 minutes per side, or until an internal temperature of 165°F is reached.
04 - During the last minute of cooking, place a slice of Swiss cheese on each patty and cover the pan to melt the cheese.
05 - Lightly toast the burger buns if desired.
06 - Spread mayonnaise and/or ketchup on the bottom buns if using. Layer with romaine lettuce, turkey patty with melted Swiss cheese, tomato slices, avocado, and optional red onion. Top with bun lids.
07 - Serve immediately alongside preferred side dishes.