1. Bring a pot of generously salted water to a boil and boil pasta until very al dente (1-2 mins less than package directions because it will finish cooking in the sauce).
2. In a pot over medium heat, sauté the garlic in butter for 1 min then add the flour; cook the flour whisking constantly for 1 min and then start gradually pouring in the milk 1/2 cup at a time, whisking so that the milk gets fully incorporated before adding more.
3. Once all of the milk is added, turn the heat to medium high and continue whisking until the white sauce starts to bubble and has thickened slightly, then add the kosher salt, pepper, nutmeg, and cayenne.
4. Turn the heat down to medium, stir in the spinach, and cook for 1-2 mins until wilted, then add the mozzarella, parmesan, and gruyere.
5. Add the pasta and cook for 2-3 more mins until pasta is desired tenderness; enjoy saucy as is, or transfer into a baking dish, top with some more cheese, and broil for 3-5 mins until top is golden brown (watch closely while you broil because things can burn easily).
