Prick the potatoes here and there with a fork and put them into the oven, straight on the wire rack, for 1 to 1 1/2 hours, depending on their size.
Take the potatoes out of the oven and let them cool down a little before you handle them. Carefully slice each 1 in half and scoop out the fluffy potato flesh into a bowl, reserving the skins.
Put the diced mozzarella into the bowl with the potato and add the 2 tablespoons of ketchup. Mix with a fork and then spoon the filling back into the potato skins.
Sit the loaded potato skins on a lined baking sheet, and dribble over some more ketchup in a gory blood-dripping kind of way. Put back in the oven to cook for 15 minutes, by which time the cheese will have melted and the potatoes be warmed through.
Recipe courtesy of Nigella Lawson(Copyright 2004, Feast, Hyperion, All Rights Reserved)