Preheat oven to 375 degrees. Cook the pasta in salted boiling water for 8 to 10 minutes, until al dente. Drain and set aside.

In a heavy saucepan over medium heat, heat milk until hot but not boiling.

At the same time, combine flour and butter in another heavy saucepan. Stir over medium heat with a wooden spoon or whisk until the mixtures has gently bubbled for 2 minutes. Be careful not to brown the flour.

Begin to add the hot milk to the flour mixture a little at a time while whisking vigorously. Continue to add the milk until it is fully incorporated. Bring the mixture to a boil, reduce the heat to low, and simmer, stirring constantly, until the mixture thickens, about 6 to 8 minutes. It will look like heavy cream. Crumble the cheese and add it to the hot bechamel sauce, whisking continuously until smooth. Add the nutmeg and pepper and stir. Remove from heat and let cool.

If using fresh spinach, blanch it 1 to 2 minutes in a little boiling water. Drain it, let it cool, and squeeze all of the water out several times. If you are using frozen spinach, let it defrost in a sieve over a bowl. Squeeze all the water out by hand.

Toss the pasta, Gorgonzola sauce, spinach, ricotta, half of the Parmesan cheese, and garlic in a large bowl until well combined. Taste and correct for salt and pepper.

Place in a 12×17-inch casserole dish and top with the rest of the Parmesan cheese. Bake for 20 to 30 minutes, until cheese browns a little. You may make this a day ahead and before baking, refrigerate well covered.