Bring a large pot of salted water to a boil. Add the fusilli and cook, stirring occasionally, until the pasta is just shy of al dente.

Stir the spinach into the pot with the pasta and cook until it just begins to wilt, about 30 seconds. Reserve some of the cooking water, then drain the pasta and spinach into a colander.

Heat the olive oil in the now-empty pot over medium-low heat until shimmering. Add the garlic, nutmeg, and cayenne, and cook until the garlic is fragrant, about 1 minute.

Whisk in the heavy cream, lemon zest, lemon juice, Parmesan, and a small portion of the ricotta until the cheese is melted and the sauce is smooth.

Return the pasta and spinach to the pot and toss gently to coat with the sauce. If the mixture seems dry, stir in the reserved cooking water a little at a time until it reaches a creamy consistency.

Season with salt and pepper to taste. Divide into bowls and dollop the remaining ricotta over the top before serving with extra Parmesan.