Preheat the grill.
Smash the anchovies into a paste with the back of your knife or a mortar and pestle. Remove the Italian sausage from its casing and add to a large bowl with the hamburger meat, smashed anchovies, onion, mint, garlic, Italian spice, salt and pepper. Mix all ingredients well.
On a clean work surface, or a cutting board covered with plastic wrap, pour out the meat mixture and shape into a flat rectangle approximately the width of the loaf of bread and about 1 inch thick. Cook's note: You can make the rectangle a bit larger than the loaf of bread since the meat will shrink slightly during cooking.
Cut the flattened hamburger in half, making 2 large patties for grilling. Place the patties on the preheated grill and cook until the ground meat reaches an internal temperature of 155 degrees F.
To prepare the bread, first saute the garlic cloves and minced onion in olive oil until they soften, about 3 to 5 minutes. Season with salt and pepper. Cut the loaf in half horizontally through the center to create 2 long pieces, top and bottom halves of the loaf. Paint the inside of each piece of bread with the oil mixture, coating them well. Remove the burgers from the heat and hold on a covered plate. Place the 2 bread halves on the coals, painted side down, to toast.
To serve, place the burger patties on half of the toasted bread, add the lettuce and tomato slices, if using, and then top with the other half of the bread and cut into 1- to 2-inch slices.