01 - Preheat oven to 375°F. Grease a 2-quart baking dish if you plan to bake the pasta.
02 - Boil macaroni in a large pot of salted water until al dente. Drain thoroughly and set aside.
03 - In a large saucepan, melt 3 tablespoons butter over medium heat. Add minced garlic and sauté for 1 minute until aromatic.
04 - Sprinkle in flour and whisk continuously for 1 to 2 minutes until a pale roux forms.
05 - Gradually whisk in whole milk and heavy cream. Cook, stirring consistently, until the mixture thickens, about 4 to 5 minutes.
06 - Remove saucepan from heat. Stir in 1 and 1/2 cups Parmesan and mozzarella until fully melted and smooth. Season with salt, pepper, and nutmeg.
07 - Add drained macaroni to the cheese sauce and blend until the pasta is evenly coated.
08 - If baking, transfer pasta mixture to prepared baking dish. In a bowl, mix panko breadcrumbs, 1/2 cup Parmesan, and melted butter. Distribute topping evenly over pasta.
09 - Bake for 15 to 18 minutes until the top is golden and cheese bubbles.
10 - Sprinkle with chopped parsley and serve warm.