Bring a large pot of water to a boil; add about 1 tablespoon salt.

Stack several leaves of spinach at a time and cut them crosswise into 1/4-inch strips. You should have about 8 packed cups of shredded spinach.

In a very large pan, heat the olive oil over medium-high heat. Add the scallions and cook, stirring, until softened, about 5 minutes. Stir in the spinach and a pinch of salt and pepper into the pan. Cover and steam the spinach until it's wilted but still bright green, about 5 minutes, stirring as needed to cook the spinach evenly.

Meanwhile, in a small bowl, stir together the ricotta, half-and-half, and nutmeg, if using, until smooth. Stir the ricotta mixture and the butter into the spinach and season with salt and pepper. Reduce the heat to medium low and simmer for 5 minutes.

Meanwhile, stir the fusilli into the boiling water and cover the pot. When the water returns to a boil, uncover the pot. Cook the pasta, stirring occasionally, until al dente, about 12 minutes.

Reserve 1/2 cup of the pasta cooking liquid. Drain the pasta well and return it to its pot over low heat. Add the spinach mixture and enough of the reserved cooking liquid to make a sauce that lightly coats the pasta. Toss thoroughly. Remove the pot from the heat and stir in the grated cheese. Transfer the pasta to a warm serving platter or individual bowls and serve.