Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the macaroni and return to the boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 8 minutes. Drain well in a colander set in the sink.
Melt 175g butter in a saucepan over medium heat and stir in salt, black pepper and mustard powder. Add the onion, stir a few times to coat with butter and cook until translucent, stirring occasionally, about 5 minutes. Gradually stir in the flour and mix until smoothly combined with the butter. Gradually pour in the evaporated milk and whisk until the sauce is thickened and bubbly, about 5 more minutes.
Stirring constantly, mix in the Cheddar cheese, processed cheese and mild Cheddar cheese until the cheeses have melted and incorporated into the sauce. Stir in the cottage cheese. Mix the cheese mixture with the cooked macaroni in a large bowl, then pour into a large foil baking tin.
Melt 110g butter in a large frying pan and toss the seasoned croutons with the butter until thoroughly coated. Press the buttered croutons into the top of the macaroni and cheese.
Bake in preheated oven until bubbly and the croutons have turned golden brown, about 30 minutes.