2) Bring a large saucepan of salted water and 1 tbsp of oil to a boil. Add the macaroni and cook until slightly al dente, about 10 mins. Drain and set aside.
3) Whisk the eggs in a large bowl until frothy. Combine the processed cheese, butter, and 475ml of half-and-half in a large bowl.
4) Add the warm macaroni, tossing until the cheese has melted and the mixture is smooth. Add the remaining 925ml of half-and-half, 300g of sharp yellow cheddar cheese, the remaining grated cheeses, and seasoning salt and pepper, tossing until completely combined.
5) Pour mixture into 4 au gratin dishes, with the remainder in a casserole, and bake for 30 mins. Sprinkle remaining sharp yellow Cheddar cheese on top and place under griller until golden brown. Serve hot.