Bring a large pot of salted water to boil. Add spaghetti and cook until al dente, about 10 minutes.
In a bowl, stir together ricotta, olive oil, lemon zest and nutmeg. Season with salt and pepper.
Drain spaghetti, reserving 1 cup cooking water. Return pasta to pot and stir in ricotta mixture, spinach and 1/2 cup pasta cooking water. Toss well, adding more pasta cooking water 1 Tbsp. at a time, if necessary. Serve hot.