Prick the potatoes all over with the tip of a sharp knife and place directly in a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour or until crisp on the outside and the inside is tender. Leave until cool enough to handle.
Cut the potatoes in half and scoop the cooked flesh into a bowl. Place the empty potato skins, cut side up, on a baking sheet. Mix the tuna, jalapeño peppers, spring onions, tomatoes and chives into the potato in the bowl. Gently fold in the soured cream, then season with salt and pepper to taste.
Spoon the filling into the potato skins, sprinkle with the Cheddar and cook under a preheated medium-hot grill for 4–5 minutes or until hot and melted. Serve immediately with a frisée salad.