Using a fork, prick each potato several times and rub over with the oil.
Heat oven to Gas Mark 4,180ºc, fan 160ºc. Place potatoes on the oven shelf and cook for 40-45 minutes, or until soft when pierced with the point of a sharp knife.
Remove from the oven and cut each in half lengthways and top with half a tub of the tuna and sweetcorn filler. Garnish with parsley sprig and serve immediately.