Mix all croquette ingredients lightly together (a few lumps are okay). Taste and adjust seasonings if necessary.
Form the croquettes into desired shapes: typical Japanese shapes are either short thick cylinders or small thick patties. Lightly pat all-over with flour (dusting off the excess), dip into egg mixture, and roll in the breadcrumbs, patting off any excess.
Quickly deep fry the croquettes in the oil until golden brown, turning once. Don't crowd the pan when doing. Drain briefly on paper towels and serve on a heated platter, garnishing the plate either with watercress or parsley sprigs. Pass Tonkatsu sauce separately. Serve piping hot.