Heat oven to 200C/180C fan/gas 6. Scrub the potatoes and dry well, then prick several times with a fork. Bake directly on the oven shelf for 1-1½ hrs, until they feel soft. If you are short of time, prick each potato with a fork, wrap in a sheet of kitchen paper and microwave on High for 8-10 mins until soft inside.
While the potatoes cook, heat the oil in a small pan and sauté the onion for 5 mins to soften. Stir in the garlic and pepperoni, and cook for 1-2 mins, then add the chopped tomatoes. Bring to the boil, then simmer for 5 mins. Season and stir in half the basil. Spoon over the split potatoes and top each one with a good handful of cheese and the rest of the basil.