Wash the spinach and cook in a pot with a little salted water for about 5 minutes until completely wilted. Drain thoroughly, then squeeze out as much water as you possibly can, a handful at a time, then again in a clean cloth. This step is not optional and cannot be rushed. Any water left in the spinach will destroy the gnudi in the pot. Chop the spinach very finely with a knife.

In a large bowl combine the chopped spinach, well-drained ricotta, egg, Parmigiano, semolina flour, nutmeg, salt and pepper. Mix until cohesive.

Dust a tray generously with semolina flour. Using two spoons or wet hands, shape the mixture into rounds or ovals roughly the size of a ping-pong ball. Roll each one gently in the flour to give it a light coating. Place on the floured tray with space between each one. Refrigerate for at least one hour, ideally two, to firm up.

Bring a large pot of well-salted water to a gentle simmer, not a rolling boil. Lower the heat so the water is barely moving. Add the gnudi in batches, do not crowd them. They are ready when they float to the surface, which takes about 2 to 3 minutes. Remove with a slotted spoon.

While the gnudi cook, melt the butter in a wide pan over medium heat. Add the sage leaves and let them crisp gently in the butter until the butter turns a light golden brown and smells nutty. Remove from heat.

Add the cooked gnudi directly into the butter and sage pan. Swirl gently to coat, do not stir aggressively or they will break. Serve immediately with extra Parmigiano grated over the top.