Heat the grill to high. Divide the ground beef into 4 portions. Salt and pepper the patties. Layer each patty with a slice of pancetta on top and bottom. Press the pancetta into each patty.
Cut the onion and mozzarella into rounds. Melt the butter and mix it with the tomato paste and anchovy paste. Cut the ciabatta buns and brush the insides with tomato-butter.
Carefully brush the grill with oil or use non-stick grill spray. Grill the patties and onions at the same time. Grill the patties for 3-5 minutes per side and the onions for 1-2 minutes per side.
Once you flip the burgers, lay the onion rounds over the top of each patty, followed by the mozzarella slices. The cheese will melt while the burgers finish cooking.
Place the buns on the open grill and toast for 1 minute. Once the buns are toasted, slather them with pesto. Layer arugula leaves, tomato slices and a patty on each bun. Top and serve!
Notes
Basic Pesto Ingredients (if making pesto from scratch):
2 cups packed fresh basil leaves
2 cloves garlic
1/4 cup pine nuts
1/2 cup freshly grated Pecorino cheese
1/2 cup extra-virgin olive oil, divided
Combine the basil, garlic, pine nuts, and pecorino in a food processor and pulse until coarsely chopped. Add 1/2 cup oil and process until fully incorporated and smooth. Season with salt and pepper.