Boil the potatoes for 10-15 minutes or until tender, drain and mash, making sure the potatoes are as smooth as possible.
Place the mashed potatoes in a large bowl and season. Add the flour and mix together to form a dough. Knead for a couple of minutes.
Divide the dough into 2 and roll each into a sausage shape. Cut the dough into 2cm pieces and using the back of a fork press gently into a gnocchi-shape.
Bring a pan of salted water to the boil.
Meanwhile, to prepare the sauce, fry the onion and garlic in the olive oil over a low heat until soft, add the tomatoes and season. Remove from the heat and stir in the basil.
Cook the gnocchi in the boiling water until they rise to the surface, drain.
Add the gnocchi to the tomato sauce, mix gently and tip into a large ovenproof dish. Top with the mozzarella and cook under a preheated moderate grill for a few minutes or until the cheese melts. Serve with a crisp salad.