Preheat oven to 375°F (190°C) and grease a 2-quart baking dish if baking.
Cook elbow macaroni in salted boiling water until al dente, then drain and set aside.
Melt butter in a large saucepan over medium heat. Add minced garlic and sauté for 1 minute until fragrant.
Whisk in the flour and cook for 1 to 2 minutes to form a roux.
Gradually whisk in the whole milk and heavy cream. Cook until sauce thickens, about 4 to 5 minutes.
Remove from heat and stir in grated Parmesan and mozzarella cheeses until melted and smooth. Season with salt, pepper, and nutmeg if using.
Add the drained macaroni to the cheese sauce and stir to coat evenly.
Transfer mixture to the prepared baking dish if baking. Combine panko breadcrumbs, grated Parmesan, and melted butter; sprinkle evenly over the top.
Bake for 15 to 18 minutes until the topping is golden and cheese is bubbling.
Remove from oven, garnish with chopped fresh parsley, and serve warm.