Preheat the oven to 400 degrees F (200 degrees C).
Bring a very large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring often, until tender yet firm to the bite, about 8 minutes.
Meanwhile, melt butter in another very large pot over low heat. Stir in flour, onion powder, garlic, Italian seasoning, mustard, salt, and pepper until a smooth paste forms. Pour in cream and milk. Add Parmesan, cottage, Cheddar, and mozzarella cheeses. Stir often, making sure to scrape the bottom of the pot, until cheeses start to melt. Increase heat to medium and cook, stirring constantly, until sauce is thick and runny.
Drain macaroni and divide between two 9×13-inch pans. Pour cheese sauce evenly over the top. Mix until sauce is spread out evenly.
Bake in the preheated oven until cheese sauce starts to brown, about 45 minutes.
You can also bake this in a single, very large roasting pan.
Serving size depends on how much people take. I have served this at a wedding with 100 people and a party of 20.
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