Heat the olive oil in a large pan and brown the garlic. Remove the garlic and add the spinach leaves. Cover and cook, medium heat, until the spinach wilts (about 3-4 minutes).
Place the spinach on a strain and remove as much liquid as possible (help yourself by pressing the leaves with a tablespoon).
Mash spinach with a blender or hand blender (just a few seconds).
Pour the ricotta cheese into a large bowl. Add spinach and mix well. Add salt, black pepper, nutmeg, parmesan cheese, egg (beaten), and flour. Mix well with a wooden spoon. Let the mixture rest for about 5 minutes.
Place 3-4 tablespoons of flour in a bowl. Use your hands to form a 2-3 inches ball of the spinach-ricotta mixture, place it in the flour bowl, and flour it completely. Place the ball on a piece of parchment paper. Repeat until you finish the mix.
Place butter and sage leaves in a large pan and melt over lo
Fill a medium-size pot with water, add a teaspoon of salt and bring to a boil. Carefully place Gnudi, one at the time, in the boiling water. As soon as the balls come to the surface, take them from the water using a skimmer and place them in the pan with the melted butter. Season gnudi carefully by moving the pan in delicate circular movements.
You may cook the gnudi up to 12 hours before serving time and then heat them in the pan with the melted butter, high heat, for a couple of minutes.