To make the winter tomato sauce heat half the oil in a frying pan and gently fry the onion and garlic over a medium to low heat for 7–10 minutes, until softened. Add the tomatoes, sugar and seasoning, and cook over a medium heat for a further 30 minutes, using a potato masher or fork to break up the tomatoes. Taste and adjust the salt and sugar if necessary, before mixing in with your chosen pasta.
Boil the potatoes in their skins in plenty of salted water until tender. Although this takes about an hour the flavour is far superior than boiling chopped potatoes and they are less watery. Peel them while they are still hot: hold on a fork in one hand and peel the skin away with a sharp knife. Pass the potatoes through a passatutto (food mill) or ricer onto a wooden board.
Add the seasoning and the flour, then the egg, and knead together to form a soft pliable dough. Bring a large pan of well-salted water to the boil.
Lightly flour your work surface. Roll an apple-sized piece of dough into a 2 cm thick length. Cut into 2 cm long pieces with a cook’s knife, flicking the gnocchi to one side as you cut them. Repeat until all the dough is used.
Cook the gnocchi in 2 batches. Drop them into the boiling water and cook for about 2 minutes. They will float to the surface when cooked. Lift them out gently, using a slotted spoon, and toss them into the tomato sauce. Serve with the mozzarella torn into pieces and the basil leaves.