In a food processor, whizz the bread to fine crumbs. Set aside, then whizz the salmon. Add the crumbs and egg white, then pulse to combine. Season. Divide into 12 balls and chill for 30 mins. Can be frozen at this stage for up to 1 month.
Heat oven to 200C/fan 180C/gas 6. Heat the oil in a non-stick frying pan. Add the fishballs and fry for 1-2 mins until lightly browned. Transfer to a baking tray, bake for 15 mins until golden.
Wipe the pan, add korma paste and onion, and cook for 5 mins, stirring regularly. Add peppers and cook for 2 mins. Add passata and lentils, bring to the boil, then simmer for 20 mins. Coat fishballs in the gravy and serve.