Place the potatoes into a saucepan and fill with enough water to cover. Bring to a boil, then reduce the heat and simmer until tender enough to easily pierce with a fork. Drain and toss gently over low heat for a minute to completely dry out. Press them through a sieve or mash with a potato masher until smooth. Season with salt, pepper and nutmeg. Mix in the butter and egg yolk, then spread out on a tray to cool.
Preheat the oil in a deep-fryer or deep saucepan to 180C. The oil is the proper temperature when a cube of bread browns in about 15 seconds.
In a large bowl, mix together the Parmesan cheese and spinach. Blend in the mashed potatoes. On a floured surface, using floured hands, roll small handfuls of the mixture into cylinders about 2cm thick and 6cm long. Tap the ends to flatten.
Season 1/2 cup of flour with salt and pepper and place on a tray. In a shallow bowl, whisk together the eggs and peanut oil. Place the bread crumbs into a separate shallow bowl. Roll the croquettes in seasoned flour, dip into the egg and then coat with breadcrumbs.
Fry the croquettes a few at a time until golden brown, 5 to 6 minutes. Carefully remove from the oil using a slotted spoon and drain on paper towels.