Boil the potato along with salt until well cooked. Allow it to cool and then remove the skin. Mash it thoroughly until there are no chunks (Alternatively, you can pressure cook the potato until cooked).
Heat 2 tbsp oil in a pan and saute onions until translucent.
Add green chillies, ginger, garlic and curry leaves and saute for a few more minutes.
Add red chilly powder, turmeric powder, coriander powder, fish masala powder (optional), pepper powder, fennel seeds powder (optional) and garam masala powder. Saute for 2 minutes.
Drain the water or oil from the tuna and add it to the pan along with salt to taste. If using fish fillets instead of canned fish, see the note section.
Mix everything well and saute the tuna for around 10 minutes. Allow it to cool.
In a large bowl, combine the cooked fish and mashed potatoes using your hands. Do a taste test and add more salt if needed.
Make lemon sized balls using your palm and flatten the balls to form cutlets of any desired shape. Place it on a plate or tray.
Beat the egg in a bowl and keep aside. Take bread crumbs in a plate and keep aside.
Take one cutlet at a time and dip it in the beaten egg and then roll it in the bread crumbs until the cutlet is fully coated with bread crumbs.
Once all cutlets have been coated with bread crumbs, heat a non stick frying pan and add oil as required for deep frying or shallow frying.
Place the cutlets slowly in the pan and fry each side on medium heat for 2-3 minutes until golden brown. Drain onto paper towels.
Serve these tasty fish cutlets with ketchup, onion salad, mint chutney or any dip.