01 - Preheat the oven to 375°F. Grease a 9x13 inch baking dish.
02 - Boil the macaroni in a large pot of salted water, cooking until al dente (1 to 2 minutes less than package instructions). Add broccoli florets during the last 3 minutes. Drain well and set aside.
03 - In a large saucepan over medium heat, melt butter. Add minced garlic and cook for 1 minute until fragrant.
04 - Sprinkle in the flour and whisk continuously for 1 to 2 minutes to create a blond roux. Do not allow the mixture to brown.
05 - Gradually whisk in the milk and heavy cream until the mixture is smooth. Bring to a gentle simmer, stirring constantly, until thickened, about 3 to 4 minutes.
06 - Add 1 1/4 cups grated Parmesan and 1 cup shredded mozzarella cheese. Stir until fully melted and sauce is smooth. Season with salt, black pepper, and nutmeg if desired.
07 - Fold the drained pasta and broccoli into the Alfredo sauce, mixing thoroughly to ensure even coating.
08 - Transfer the pasta mixture to the prepared baking dish. Evenly sprinkle over 1/2 cup Parmesan and 3/4 cup mozzarella cheese.
09 - Bake uncovered in the preheated oven for 20 to 25 minutes until the surface is golden and bubbling.
10 - Allow the bake to rest for 5 minutes. Optionally, garnish with freshly chopped parsley before serving.