Mix breadcrumbs and milk in a bowl and soak for 5 minutes until absorbed.
Add beef, garlic, tomato purée, Worcestershire sauce, and parsley, then season and mix well.
Shape the mixture into six large flat patties.
Press a slice of pancetta onto both sides of each patty.
Fry patties on high heat for 4 minutes per side until pancetta is cooked.
Combine mayonnaise, mustard, and tarragon in a small bowl.
Preheat high grill and toast the sliced bread rolls.
Assemble by spreading mayonnaise on rolls and adding spinach, patties, relish, and cheese.
Serve the burgers immediately with sliced gherkins.