Chop up the tuna as much as you can.
Add the finely chopped garlic and parsley together with the grated onion. Then, add the egg, salt and pepper and mix it all very well with a fork or your hands.
Add the breadcrumbs and continue working with your hands until the mixture is a compact dough that you can shape. Let the dough rest in the refrigerator for an hour or two.
Finally, shape the hamburgers, roll them in breadcrumbs and fry them in hot oil until golden brown on both sides.