Heat oil in a pan over medium-high heat. Add onions, ginger, green chilies and curry leaves. Sauté, stirring occasionally, for 2-3 minutes or until onions start to brown. Add ground turmeric. Fry, stirring, for a few seconds.
Add drained tuna. Sauté, stirring occasionally, for 2-3 minutes. Add ground black pepper and heat through. Remove from heat. Leave to cool.
Add potatoes and garam masala to the cooled tuna mixture. Season with salt. Make 26 small balls out of the mixture and press down lightly to form round or oval shaped patties or cakes. Set aside.
Place eggs and breadcrumbs in two separate bowls. Dip each cake or cutlet in the egg and then roll in breadcrumbs.
Meanwhile heat oil for deep frying in a large pan over medium-high heat. Add cutlets (in batches) to the hot oil and deep fry. Flip over to cook both sides. When the cutlets turn golden, remove them from oil with a slotted spoon, and drain on an absorbent towel. Serve with onion salad (challas) and ketchup.
Note: If using fresh fish, steam the whole fish and separate into tiny pieces. If the fish has bone, debone them. Make sure you remove all the tiny bones.