Mix the minced meat with the egg, tomato paste, chopped onion and breadcrumbs well and form 6 bread-sized meatballs. Place a slice of cheese between two meatballs and press the edges tightly.
Prepare on the grill or in the pan and place on the roasted burger buns, which are topped to taste. For the extra portion of cheese, melt an extra slice of cheese on the meatball.