For the herb mayo: Combine the mayonnaise, Dijon mustard, fresh lemon juice, tarragon, chives and parsley in a medium bowl. Season with salt and pepper. Stir until well blended.
For the burgers: Divide the ground beef into four equal portions. Gently form each portion into patties 1 to 1 1/2 inches thick and 3 to 4 inches in diameter. Do not over work or tightly pack the meat or the burgers will not be tender. Place on a plate or sheet tray and bring to room temperature.
Heat a large saute pan over medium heat. Take the slices of pancetta and place them in the pan. Begin to render the pancetta on both sides, 3 to 4 minutes, rotating the pancetta until golden brown and crispy. Transfer to a plate lined with paper towels and let cool. Reserve the pancetta fat in the pan and use as a fat substitute instead of butter to toast the buns in.
Brush a griddle with the pancetta fat and preheat to medium-high heat.
Sprinkle the burgers on each side with salt and pepper. Place the burgers on the brushed and heated griddle and cook 4 to 5 minutes each side for medium rare.
Brush the buns with pancetta fat and toast on the griddle until golden brown.
When the burgers are almost finished, top each with 2 slices of the cheese and cook for 2 to 4 minutes. Take the burgers off the griddle and let rest.
Mix the tomato slices with the vinegar and season with salt and pepper.
Spread the herbed mayonnaise on the buns. Top with lettuce, tomato, a burger and a piece of pancetta.