Bring a pot of water to boil and salt it generously. Add the spinach and stir. Boil until the leaves are very soft and tender. Drain the spinach.
Let the spinach cool to the touch and squeeze out the excess water. Finely chop the spinach and place it into a large mixing bowl.
Add the ricotta and a generous pinch each of salt, pepper and nutmeg. Also add the egg and grated cheese to taste. Stir all together and season it again to taste.
Mix in flour until you achieve a soft but firm dough that can be rolled into balls.
Scoop up a small palmful of dough and roll it into a ball (about 2-inches or 5cm in diameter). Toss the ball in flour until it is completely coated and set aside.
Bring a large pot of water to boil and salt it. Add the butter and sage leaves into a large skillet over low heat to brown the butter.
Gently drop the gnudi one at a time into the boiling water. Don't overcrowd the pot.
After 30 seconds, gently stir the gnudi. When they float to the surface, they are cooked.
Using a slotted spoon, transfer them into the butter sauce. Turn the heat up to medium and gently swirl the pan around.
Continue to cook them for 2-3 minutes so they absorb some sauce. Serve immediately, drizzled with extra butter from the pan.
