Start by squeezing all excess water from the thawed spinach using your hands or a clean towel. This step is crucial because too much moisture can make the gnudi fall apart. Once dry, chop the spinach very finely so it blends evenly into the mixture.
In a large bowl, combine the chopped spinach, ricotta, egg, parmesan, salt, pepper, and nutmeg. Mix everything until smooth and thick. Add the flour gradually and stir until the mixture holds together but still feels soft.
Using a spoon or small scoop, portion the mixture into small balls. Gently roll them between your hands to form smooth dumplings. Place them on a tray and, if possible, chill for about 20–30 minutes to help them firm up.
Bring a pot of salted water to a gentle boil. Drop the gnudi in small batches and cook until they float to the surface, which takes about 3 minutes. Carefully remove them with a slotted spoon and set aside.
In a skillet, melt butter over medium heat until it starts to sizzle. Add sage leaves and cook until crispy and fragrant. Place the gnudi in the pan and gently turn them to brown on all sides for a golden finish.