Combine the zest, lemon juice, mascarpone, salt, pepper, and nutmeg in a bowl. Whisk to combine.
Bring a pot of water to boil and salt generously. Cook the pasta until al dente (still firm when bitten), taking it off the heat about 1 to 2 minutes before you would normally remove it. Drain, but first save about 1/2 cup of the pasta water and set aside.
Return the pasta to the pot, and set over low heat. Stir in the mascarpone sauce. Add the spinach and toss so that the spinach begins to wilt. Add about 1/4 cup of the pasta water (more if necessary) to keep the sauce fluid but not too watery. Continue to cook and toss until the spinach is cooked. Serve immediately.