Place the potato into a small pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until tender, about 20 minutes. Drain and allow to steam dry for a minute or two, then mash the potato with a potato masher or fork in a large bowl.
Mix the tuna, egg, onion, Dijon mustard, bread crumbs, garlic powder, Italian seasoning, cayenne pepper, salt, and pepper into the mashed potato until well-blended. Divide the tuna mixture into 8 equal portions and shape into patties.
Heat the olive oil in a skillet over medium heat. Pan fry the tuna patties until browned and crisp, about 3 minutes on each side.