The kumquat is an unusual citrus fruit: unlike most other types, its rind is sweet while the flesh is tart. The whole fruit is edible, though most people prefer to remove the seeds. They add a bright tart note to this easy dish of yellowtail carpaccio (hamachi carpaccio).
Make the kumquat oil first. Put four of the kumquats in a mortar and roughly crush. Put the crushed kumquats in a bowl and add the olive oil. Thinly slice the remaining kumquats, removing the seeds as you go. Put the sliced kumquats in the oil and leave at room temperature for at least 30 minutes.
Roughly crush the pink peppercorns in the mortar.
Use a very sharp knife to slice the yellowtail across the grain into 5mm (¼in) thick pieces. Divide the yellowtail slices into four portions. Lay a sheet of cling-film smoothly on the work surface. Place one portion of the fish slices in a 15cm (6in) circle over the cling-film, leaving space between each piece. Cover with another sheet of cling-film then use the flat side of a meat mallet to very gently pound the slices, starting at the centre and working outwards, until the pieces are very thin and touching each other, and the diameter of the circle is enlarged to about 18cm (7 inch). Occasionally lift the top layer of cling film from the fish then smooth it back over the pieces. Flip the package over, lift the cling-film on the other side and again, smooth it over the fish. Repeat with the remaining slices. Stack the yellowtail - still in the cling-film sheets - on a flat tray and refrigerate until ready to serve.
Peel away the top sheet of cling-film from one portion of yellowtail. Invert the fish onto a plate and peel away the other sheet of cling-film. Repeat with the remaining portions of fish.
Drizzle some of the kumquat oil over each portion, then add several kumquat slices. Sprinkle with pink peppercorns and rough-flaked salt, then garnish with the herbs.
The remaining kumquat oil can be strained (the fruit being discarded) then stored in a glass jar in the refrigerator. It keeps for at least a week.