Heat oil in a small frying pan over medium heat. Add onion. Cook for 2 minutes or until soft. Set aside to cool for 5 minutes.
Place bread into a food processor. Process until fine crumbs form. Place tuna, lemon juice, tomato sauce, Worcestershire sauce, 1 egg, parsley, and salt and pepper into a large bowl. Add onion and 2 cups of breadcrumbs. Mix with clean hands until well combined (see hint below).
Form 1/4 cup of tuna mixture into a patty. Repeat with remaining mixture to make 12 patties. Place flour onto a plate. Beat remaining 2 eggs together in a shallow dish.
Line a tray with paper towels. Pour enough oil into a frying pan to cover the base. Heat over medium heat until hot. Coat patties in flour then dip into egg. Cook patties, 3 at a time, for 3 minutes each side or until golden. Place onto prepared tray. Cover to keep warm while cooking remaining patties.
Place patties onto serving plates. Serve with tzatziki, salad and chips.