Fry onion on a lowish heat so they soften, rather than go brown and crispy. Boil your potatoes until soft. Sift the flour and salt into a separate bowl then add the water slowly, whisking all the time until you have a smooth batter. Set aside.
Mash your potato. Add the onion and tin of sardines then mash again.
Put some flour on your work surface then dust some on your hands and form the mash mixture into patties, dust them off with some flour which will help the batter stick to them.
Heat up some oil in a frying pan and get it hot. Dip your patties into the batter mixture then pop them into the frying pan and fry on a medium heat until golden brown.
These are great if you lift the top of the batter off and sprinkle on some vinegar to the mash.