Boil and mash the potatoes. Let them cool slightly.

In a large bowl, combine the fish mince, mashed potatoes, grated carrot, egg, and flour.

If using, add the finely chopped onion.

Season the mixture generously with salt and pepper.

Mix everything together until just combined. Don't overmix.

Shape the mixture into 4-6 patties.

Heat a generous amount of oil in a frying pan over medium-high heat.

Carefully place the patties in the hot oil (do not overcrowd the pan) and fry for 4-5 minutes per side, or until golden brown and cooked through.

While the patties fry, mash the avocado in a small bowl. If using, stir in sour cream or yogurt and season with salt and pepper.

Remove patties from the pan and drain on paper towels.

Serve the fish cakes hot with a dollop of avocado smash and an optional lemon wedge.