Melt the butter in a skillet over medium-low heat. Next, add the leeks with a good pinch of salt and cook slowly, stirring from time to time until completely tender, about 8-10 minutes. (They should not brown at all, lower the heat if necessary.) Turn off the heat and let cool.In a large bowl, combine the ricotta cheese, cooled leeks, Parmigiano-Reggiano cheese, fresh nutmeg, eggs, and bread crumbs then taste for seasoning. Lastly, add the flour.Place the extra cup of flour in a shallow dish such a pie pan and have a fork nearby. Using two spoons to help you, create walnut-sized pieces grabbing each piece with one spoon and using the other to slide it into the flour. With your fork, cover each piece with the flour, then roll each one in your hands to create a round, even ball. Shake off any excess flour. (Using the spoons and fork will keep your hands cleaner and make the process easier.) Continue until you have made all of your gnudi. The recipe should yield about 38-40 gnudi.Meanwhile, bring a large pot of water to a boil, add salt, then reduce to a simmer.Melt the butter over low heat in a large skillet and throw in the sage leaves. Do not let the butter brown. Add some of the boiling water as necessary to create a creamy butter sauce. Turn off the heat and taste for salt.Delicately place the gnudi in the simmering water and gently stir once or twice to make sure they are not sticking. Wait until they float, which should take 1-2 minutes. After they float, leave them in the water another 30 seconds to a minute until cooked through. Turn the heat on low under your butter sauce. Gently scoop the gnudi into the butter sauce and lightly shake the pan to coat them in the butter. Turn off the heat and serve immediately with extra Parmigiano-Reggiano cheese on top.