Place the ricotta in a strainer over a bowl for a few hours (or overnight), and keep in the fridge.
Boil a large bunch of fresh spinach (leaves only) for a few minutes in salted water, strain very well, and then transfer to a chopping block. Mince with a large knife and again press out excess water, this time in a mesh strainer if possible. You want about 1 cup/200 grams of cooked spinach.
Combine all the gnudi ingredients in a bowl. Now test the consistency. You are aiming for a mixture that is firm enough to shape but is not dry. You might have to add more flour to arrive at the right texture.
Test by rolling a small portion of the mixture in your hand: it should be workable yet sticky. Set a large pot of water to boil. Lightly dust a plate with flour. Form the gnudi into either round balls or oval shapes (it helps to flour your hands), place them on the plate and gently roll them in the flour. Lower the heat on the water until you have a mild boil, between a simmer and a rolling boil. Cook the gnudi in a few batches for about 4 to 5 minutes. They are done when have popped up to the water’s surface and are firm to the touch. Gently scoop them out with a slotted spoon.
Heat Napoletana sauce in a pan and swirl in the sage leaves. Transfer the cooked gnudi to this pan as they finish cooking, coat them with the sauce, and keep them warm while you proceed with the remaining batches. Top with grated Parmesan and serve.
Thanks @italymagazine for the recipe, I slightly changed it to vegetarian diet.