Preheat Oven: Start by preheating your oven to 350°F (175°C).
Cook the Pasta: Boil the macaroni in a large pot of salted water until al dente, about 8 minutes. Drain and set aside.
Mix Wet Ingredients: In a large bowl, whisk together milk, heavy cream, eggs, mustard, salt, pepper, and nutmeg.
Saute Aromatics: In the same pot, melt butter over medium heat. Add onions and garlic, cooking until soft.
Combine: Add the cooked macaroni back to the pot with the onions and garlic. Pour the milk mixture over the macaroni. Add three cups of cheese and stir until well combined.
Bake: Transfer the mixture to a greased baking dish. Top with remaining cheese and breadcrumbs.
Bake Covered: Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for another 30 minutes until the top is bubbly and golden brown.
Rest: Let the dish cool for 10 minutes before serving.