In a small bowl combine mayonnaise and 2 tablespoons pesto. Chill until ready to use. In a medium sized mixing bowl combine ground beef, pancetta, minced garlic, salt, pepper, and remaining pesto. Mix well and shape into four patties.
Grill burgers over medium heat for about 5-7 minutes on each side or until a thermometer reads 160 degrees. Top burgers with mozzarella cheese and grill until cheese is melted. Spread cut sides with pesto mayo and top with red onion tomato, and arugula.