In a large bowl combine the ground beef, Italian sausages, a beaten egg and salt. Do not overmix. Divide into 6 equal parts. I use a scale but it's not necessary. Your patties will be approximately 180g/6 oz each. Then roll them into balls and using any flat surface like the bottom of a frying pan press them down to approximately 1 inch thick. Your hamburger patties should be slightly larger in diameter than your buns as they will shrink when cooked.

Then press an indentation in the middle of each burger with your thumb, this will keep the patties flat when cooking, otherwise they have a tendency to puff up in the middle. Arrange the burgers in a single layer on a parchment lined pan or a cutting board and chill them in the fridge for at least 1 hour or in the freezer for 30 minutes. (This step helps patties to keep their shape on the grill.)

Meanwhile cook the prosciutto. Preheat the oven to 200C/400F. Lay the prosciutto slices on parchment lined baking dish and cook for 6-8 minutes till crispy. Don’t lay them too flat, just throw them on the pan and leave them as they land. This way the prosciutto slices will crisp up in crumbly clumps. They will get crispier as they cool. (This step can be done a day ahead)

Combine the mayonnaise with the grated garlic and grated Parmesan cheese, set aside till needed. Prepare the toppings by slicing the onions and tomatoes.

Grill your burgers for 5 minutes on each side. Cook time may vary depending on the level of heat from the grill. Be alert to any flare-ups from burning grease if using a BBQ. Do not press on your burgers while they are on the grill or all the juices will leak out. Add sliced cheese on top of each burger 1 minute before you take your hamburgers off the grill, cover with a lid and let the cheese melt. Just before the cheese is fully melted, add the sliced buns to the grill to lightly toast them. Watch closely as they burn quickly, especially if using brioche buns due to sugar content. (If you are worried about your meat not being done, use a digital meat thermometer to check the internal temperature. It should read 160F/71C.)

To assemble the burgers spread a tablespoon of basil pesto on the bottom half of each bun, then top with lettuce, which will serve as a barrier to keep the bun from going soggy, then top with the burger patty, sliced red onion if using, crispy prosciutto and a slice of tomato. Spread a tablespoon of garlic Parmesan mayo on the top half of the bun and place on top of the burger. Enjoy immediately.