Place the sweet potatoes, whole and with their skins on, in a baking tray. Season with coarse sea salt and cover with tin foil. Place in a pre-heated oven at 190ºC and cook for 45 minutes (if they are on the large side, adjust times accordingly). As soon as they are cooked through, leave to cool and chop into slices.

Fillet the fish and chop the flesh very finely.

Season the chopped mackerel with the lemon juice and zest, the chopped chives, fleur de sel and olive oil.

Serve the tartar on top of the slices of sweet potato and garnish with chive flowers.