Preheat the oven to 450 degrees F.
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.