Wash the potatoes, place on a baking tray and cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 35-45 minutes. Peel onions and cut into thin rings.
Wash the chilli, cut into thin rings. Wash coriander, shake dry and finely chop leaves of 4 stems. Wash the lime, grate dry and cut in half. Cut 1/2 lime into slices, squeeze the other half.
Put the tuna fish in a sieve and let it drain. Mix yoghurt, chili and 1-2 teaspoons lime juice and season with salt, pepper and a little sugar. Fold in tuna, chopped coriander and onion rings.
Remove potatoes from the oven and cut lengthwise. Fill potatoes with tuna salad and garnish with remaining coriander and a lime wedge.