Peel and grate the cooled, parboiled baking potatoes into a mixing bowl. Add the finely sliced onion, season with salt and pepper, and mix well until combined.

In a non-stick frying pan, heat vegetable oil over medium-high heat until shimmering.

Divide the potato mixture into four equal portions. Form each portion into round patties.

Fry the patties in the hot oil for about 4-5 minutes until golden-brown.

In a separate bowl, combine the brown crab meat, finely chopped green chilli, and mayonnaise. Mix well.

On each warm rosti, top with the crab mayonnaise, sprinkle white crab meat, and add land cress and chives. Serve with a lemon wedge.