Heat the oven to 200C/fan 180C/gas 6. Prick the potatoes all over with a fork and sprinkle with a little sea salt. Bake until tender, approx 1 hour.
Mix the tuna, onions, chives, mayo and half the cheese. Halve 2 cooked jacket potatoes then scoop some out into the tuna bowl, leaving a 1 cm shell.
Roughly mix the potato and tuna together, season well, then pile back into the shells. Sprinkle over the rest of the cheese and put in the oven until bubbling and golden. Serve with a green salad.