Put the potatoes in a saucepan and cover with water. Cover the pan and bring to the boil. Reduce the heat and simmer for about 20 minutes until cooked through – a potato pierced with a blunt knife should slide off the blade. Drain and leave until cool enough to handle.
Peel the warm potatoes, then pass through a mouli or ricer to make a light, smooth mash. Transfer to a large bowl, add the nutmeg and season with salt and pepper. Sift in 150 g (5 oz) flour and add the spinach. Break in the eggs, then gently but quickly work the mixture through your fingers until it reaches a lumpy breadcrumb consistency. Tip the mixture on to a work surface and gently knead to a soft, smooth and pliable dough, adding more flour if too wet. Be careful not to overwork, or the gnocchi will lose their light quality.
Divide the dough into 3 pieces and roll each piece into a finger-thick length. Using a sharp knife, cut into 2.5 cm (1 inch) pieces. Transfer to a floured tray and leave to rest for 10–20 minutes. Meanwhile heat the butter ingredients in a frying pan, until the butter has melted and leave to infuse.
Bring a large saucepan of salted water to the boil. Add the gnocchi, return to the boil and cook for 3–4 minutes, or until they float to the surface. Remove with a slotted spoon into a frying pan with the melted butter. Toss well and serve immediately.