Cook pasta according to package directions.

While the pasta cooks, heat butter in a pan and add the mushrooms and salt. Sauté until soft, about five minutes. Add the spinach and continue to sauté until wilted and the liquid from the mushrooms has evaporated.

Add garlic and pepper, and sauté an additional two minutes

Add the flour, mixing to incorporate. Add the milk half a cup at a time, stirring frequently as the sauce thickens.

Stir in the cheese. Once melted, add the sage and nutmeg.

Drain the cooked pasta and return to the pot.

Add the sauce to the pasta, mix to incorporate, and serve.