Peel and chop the potato and sweet potato. Boil them in a saucepan full of salted water for 20 minutes.
Drain the potatoes, mash with a fork in a bowl and set aside.
Heat the butter in a frying pan. Squeeze lemon juice over the salmon and grill until the salmon is no longer pink inside. Flake onto a plate and set aside.
Add the finely chopped parsley, spring onions, passata and mayonnaise to the mashed potatoes. Season to taste with salt and pepper.
Gently mix in the flaked salmon.
Form the mixture into golf ball sized balls (approximately 1 ½ Tablespoon per ball).
Set out 3 shallow bowls, one with flour, one with the egg (lightly beaten) and one with breadcrumbs.
Dip each fish ball into the flour, then the egg and shaking off the excess, then into the breadcrumbs until coated.
Heat the cooking oil in a fry pan. When the oil is hot, shallow fry the salmon balls, turning at regular intervals until the outside is crispy and golden. Serve with some pasta, rice or vegetables for a nutritious dinner.