Combine the zest, juice, mascarpone, salt, pepper and nutmeg in a bowl. Whisk together until smooth.
Bring a large pot of water to boil and add a generous amount of salt. Boil the spaghetti until al dente - removing pasta from heat one or two minutes earlier than you normally would. Drain the pasta - reserving 1/2 cup of the cooking water.
Heat the butter and olive oil in a large pot. Add the garlic and saute until tender. Add the spinach and fry until wilted. Add the lemon/mascarpone mixture, the pasta, and as much of the reserved water as necessary to make the sauce fluid but not watery.