Heat the oil in a large frying pan over a high heat and wilt the spinach, about 2-3 mins. Tip the spinach into a colander and press out any excess liquid using the back of a spoon. Leave to cool completely. Squeeze out any remaining liquid with your hands, then finely chop the spinach.
Tip the chopped spinach into a bowl with the ricotta, breadcrumbs and parmesan. Add the egg and semolina or flour. Season and grate over some nutmeg. Stir to form an evenly combined, sticky mixture.
Put a couple tablespoons of flour on a large tray and shake gently to spread the flour out. Spoon around ½ tbsp of the spinach mixture onto the floured tray and roll into a ball. Repeat with the rest of the spinach mixture, transferring the balls to a second clean baking tray as you go.
Heat the oven to 140C/120C fan/gas 1. Bring a large pan of salted water to the boil. Drop in the malfatti (you may need to do this in batches) and simmer for 2 mins, or until they float to the surface. Scoop the malfatti out with a slotted spoon and drain well. Transfer to an ovenproof serving dish and keep warm in the oven.
Make the sage butter by melting the butter in a small pan. Add the sage leaves and fry gently, stirring often until crisp. Pour the sage butter over the malfatti and top with more grated cheese.