Heat oven to 200C/180C fan/gas 6. Prick the potatoes, place on a baking tray and bake for 45 mins or until really soft.
Meanwhile, heat the oil in a frying pan, add the onion and cook gently for 8 mins until softened. Stir in the garlic, then tip in the mince and stir to break up. Cook over a high heat until any liquid has evaporated and the mince is browned, about 10 mins. Pour in the passata, then fill the carton a quarter full of water and tip that in too. Add the barbecue sauce and cayenne, then lower the heat and simmer gently for 15 mins, adding a little extra water if needed. Taste and season.
When the potatoes are soft, split them down the centre and spoon the mince over the top. Add a dollop of soured cream and a sprinkling of chives.