01 - In a large bowl, combine ground turkey, chopped onion, garlic, Worcestershire sauce, Dijon mustard, salt, pepper, parsley, and breadcrumbs. Mix gently until just combined; do not overmix.
02 - Divide the mixture into 4 equal portions and shape each into a patty about 0.5 inch thick.
03 - Preheat a grill, grill pan, or skillet to medium-high heat.
04 - Cook the turkey patties for 5 to 6 minutes per side, or until fully cooked through (internal temperature should reach 165°F).
05 - In the last 2 minutes of cooking, place a slice of cheese on each patty and cover to melt.
06 - While the patties cook, slice the buns and butter them lightly. Toast the buns on the grill or skillet until golden.
07 - Spread mayonnaise, ketchup, and mustard on the bottom buns. Layer with lettuce, tomato, and onion slices. Add the cheesy turkey patty, then close with the top bun.
08 - Serve immediately with your favorite sides.