1.Combine beetroot and vinegar in a glass or ceramic bowl. Season. Set aside for 10 minutes. Drain.
2.Meanwhile, place potato, salmon, ginger, coriander and breadcrumbs in a bowl; mash with a fork until well combined. Season. Using damp hands, shape mixture into 4 patties. Heat oil in a frying pan over moderate heat. Cook patties for 2 minutes on each side or until golden and heated.
3.Place bread roil bases on serving plates. Top with rocket, patties, beetroot, extra coriander and bread roll tops. Serve.
Make ahead: Prepare patties a day ahead. Cover, then refrigerate. If time permits, refrigerate patties for 30 minutes before frying. This helps them hold their shape. Try tuna instead of salmon.