Cook the sweet potato in a large saucepan of salted boiling water for 7-8 minutes or until tender. Drain and refresh under cold running water. Transfer to a large bowl.
Use a potato masher or fork to mash the sweet potato until coarsely mashed. Add the tuna, egg and breadcrumbs, and stir until well combined. Season with pepper. Divide the tuna mixture into 8 equal portions. Use wet hands to shape portions into 8cm patties.
Heat oil in a large non-stick frying pan over medium-high heat. Add the patties and cook for 3-4 minutes each side or until golden brown. Remove from heat. Divide among serving plates and season with salt and pepper. Serve immediately with bread rolls.