Sauce: Peel tomatoes, remove seeds and dice finely. Heat 1 tbsp. butter and fry finely chopped onion. Add tomatoes, spice with salt and pepper and simmer the sauce until creamy, stir on and off.
Cook the whole, unpeeled potatoes in boiling salted water for about 25 minutes or until tender. Drain. If cool enough to handle but still warm, peel the potatoes and pass them through a mouli or ricer. It is important, the potatoes are still warm at this stage.
Place the potatoes on a clean work surface, add salt and flour. Fold the mixture together working towards the centre, gradually adding more flour if the mixture is too wet. Be careful not to add too much flour as the more flour added the firmer the gnocchi will be. Once the gnocchi mixture has come together rest for 5 minutes.
Cut the gnocchi mixture in quarters and on a lightly floured surface roll each piece into a sausage shape about 2cm in diameter, then cut into 3cm pieces.
Cook the gnocchi in gently boiling salted water for 2-3 minutes or until they float.
Drain the gnocchi. In a pan heat 1 tbsp. butter and sautée hackled sage. Add gnocchi for some minutes.
Serve gnocchi with tomatoe sauce, sprinkled with parmesan.