1 tablespoon oil
1 onion, chopped
1 celery stalk, chopped
2 carrots, chopped
2 (14-ounce) cans crushed tomatoes
1 teaspoon sugar
2 1/4 pounds russet potatoes
2 tablespoons butter
2 cups all-purpose flour
2 eggs, beaten
2 tablespoons finely grated mozzarella cheese, to serve
To make the tomato sauce, heat the oil in a saucepan, add the onion, celery, and carrot, and cook for about 5 minutes, stirring. Add the tomatoes and sugar, and season. Bring to a boil, then reduce the heat to very low and simmer for 20 minutes. Cool a little, then process, in batches, in a food processor To make the potato gnocchi, peel the potatoes, chop roughly, and boil until very tender. Drain and mash until smooth. Using a wooden spoon, stir in the butter and flour, then beat in the eggs. Leave to cool.
Turn out the gnocchi mixture onto a floured surface and divide into four. Roll each into a sausage shape. Cut into short pieces and press each piece with the back of a fork.
Cook the gnocchi in batches in a large saucepan of boiling salted water for about 2 minutes, or until they rise to the surface. Using a slotted spoon, drain the gnocchi and transfer to bowls. Serve with the tomato sauce. Sprinkle with mozzarella cheese.