First place the unpeeled potatoes in a pan of slightly salted water, bring to the boil and cook until the potatoes are tender. Keep the potatoes whole - if they are cut, they absorb water. If you prefer, you can bake the potatoes in the oven.
Meanwhile, make the filling. Heat the olive oil in a pan and sweat the onion and garlic. Then add the mushrooms and stir-fry for a minute. Season with salt and pepper, add the herbs and cook for a few minutes until the mushrooms are soft. Set aside and leave to cool.
To make the gnocchi, once the potatoes are cooked, drain and leave to cool. When cool, peel and discard the skin and mash the potatoes.
Place the mashed potato in a large bowl together with the flour and cornflour. Add the eggs and salt and mix well until you get a smooth, but slightly sticky dough.
Place on a floured worktop and with a rolling pin roll out to a thin sheet. With a 6cm/2½in pastry cutter, cut out approximately 20 rounds. Place 1 tsp of the filling in the centre of half the round shapes. Place the empty ones over the top and with your fingers press down the edges.
Bring a large saucepan of slightly salted water to the boil, then drop in the gnocchi. When they come up to the surface, cook for a further 2 minutes (remember these are filled gnocchi and much thicker than normal gnocchi, so they need a little longer to cook through).
Meanwhile, in a pan melt the butter, add the sage and gnocchi, then mix in the parmesan cheese. Mix well together. Drizzle some balsamic vinegar over the top and serve.