Mix the lemon zest, lemon juice, mascarpone, nutmeg, salt and pepper in a bowl.
Boil a pan of water and add some salt. Cook the spaghetti until it is al dente. Drain and keep about 120 ml of pasta water.
Return the pasta to the pot, and set over low heat. Stir in the mascarpone sauce. Add the spinach and stir lightly so that the spinach begins to wilt.
Add the pasta water carefully about 1/4 cup to keep the sauce fluid but not too watery.
Continue to cook and toss until the spinach is cooked. Add the hazelnuts and stir to combine.