Heat the oven to 450 degrees F. Oil a 9-by-13-inch baking dish.

In a large pot of boiling, salted water, cook the rigatoni until almost done, about 12 minutes. Drain. Put the pasta in the prepared baking dish and toss with 1 tablespoon of the oil.

Meanwhile, squeeze as much of the water as possible from the spinach. Put the spinach in a food processor and puree with the ricotta, 3 tablespoons of the Parmesan, the nutmeg, salt, and pepper. Stir in half the fontina.

Stir the spinach mixture into the pasta. Top with the remaining fontina and Parmesan. Drizzle the remaining 2 tablespoons oil over the top. Bake the pasta until the top is golden brown, 15 to 20 minutes.