Bring 4 quarts of water to a boil in a large pot. Add the pasta and salt, cooking until al dente. Just before draining, stir the spinach into the pot and cook until wilted, about 30 seconds. Reserve 1 cup of the cooking water, then drain the pasta and spinach and return them to the pot.

While the pasta cooks, heat the oil, garlic, nutmeg, and cayenne in a small saucepan over medium heat. Cook until the garlic is fragrant, about 1 minute. Remove the pan from the heat and whisk in the ricotta, cream, lemon zest, lemon juice, and salt until the sauce is completely smooth.

In a small bowl, whisk the ricotta, oil, pepper, and salt for the topping together until smooth and creamy.

Pour the warm ricotta-cream sauce and the Parmesan over the cooked pasta and toss thoroughly to combine. Let the pasta rest for 2 to 4 minutes, tossing frequently, until the sauce thickens slightly and evenly coats the noodles. If the mixture appears too thick, stir in the reserved cooking water a splash at a time until the desired consistency is reached.

Transfer the pasta to a large serving platter and dot the surface evenly with the seasoned ricotta mixture. Serve warm, with extra Parmesan on the side.