Preheat the oven to 375 degrees; butter a 9-inch square baking pan.
Bring a large pot of water to a boil (over high heat); add macaroni and cook 8-10 minutes or until al dente; drain and rinse with cool water.
Place drained macaroni in a large mixing bowl; drizzle with the oil.
In a small saucepan over med-high heat, warm the milk.
In a large saucepan over med-low heat, melt 4 tablespoons butter; sprinkle in the flour and whisk constantly until the flour is absorbed and the mixture is bubbling gently and is lightly golden (2-3 minutes).
Gradually add in the warm milk, whisking constantly.
Increase the heat to medium and bring to a simmer, stirring with a wooden spoon until smooth and slightly thickened.
Add in the cheddar, American cheese and fontina; remove pan from the heat.
Whisk the mixture until the cheese is almost completely melted, stir in the salt, pepper, and mustard.
Pour the sauce over the macaroni; stir to combine.
Transfer mixture to the prepared pan.
In a small bowl, combine the 1/4 Celsius shredded cheddar and the breadcrumbs; sprinkle evenly over the macaroni.
Cut the remaining teaspoon of butter into small bits and dot the casserole.
Bake on the middle rack for 20-25 minutes or until bubbling and a crust begins to form.