1. In a medium bowl, whisk the mayonnaise with the ketchup, red wine vinegar, onion, parsley, tarragon and Worcestershire sauce. Cover and refrigerate.
2. In a large skillet, cook the bacon over moderately high heat, turning once, until crisp, about 6 minutes. Drain and cut the bacon into large pieces.
3. Light a grill and fill a large bowl with ice water. Gently mix the ground chuck with the ground sirloin, salt and pepper. Form the meat into six 4-inch patties, about 1 1/4 inches thick. Submerge the patties in the cold water and let soak for 30 seconds. Immediately transfer the burgers to the grill and brush with some of the melted butter. Grill over high heat for 9 minutes for medium-rare meat, turning once or twice and brushing occasionally with butter. Top the burgers with the cheese during the last minute of grilling and let melt.
4. Spread the tarragon dressing on the buns. Set the lettuce leaves and tomato slices on the bottom halves and top with the burgers, red onion and bacon. Close the burgers, cut in half and serve right away. —Laurent Tourondel