This to me is Americana on a plate. I grew up eating this dish and when I became a Dad I made sure my child would also grow up eating this as well
We sometimes add in some fried off bacon to the cheese sauce
Boil the pasta in salted water under nearly done (about 6 minutes). You still want it al dente as the paste will be baked. When ready, rinse in cold water to stop the pasta cooking and drain.
Put the oil and butter in a saucepan large enough to contain the cheese sauce and melt the butter
Add the onions and fry on a medium heat for 5 minutes. Add the garlic and English mustard and fry for a minute.
Add the flour and stir constantly to make a thick paste. Cook the paste for a few minutes
Gradually pour in the milk and keep stirring as to remove lumps. The sauce should be looser/thinner than a standard white sauce. If you need more milk as it's too thick then gradually add some extra
Remove the sauce pan from the heat and add the grated Cheddar cheese. Stir until the sauce is smooth and thickened
Butter a baking dish large enough to hold the pasta and cheese sauce. Add the pasta and then pour over the cheese sauce to cover. Give the pasta and sauce a quick stir to makwe sure it's all incorporated
Blitz the sliced bread in a blender to make fresh bread crumbs.
Melt the butter in a saucepand and when melted, pour in the breadcrumbs, remove from the heat and stir until the butter has coated the breadcrumbs evenly
Sprinkle the breadcrumbs over the pasta and cheese sauce dish. Cover with tin foil
To serve, heat the oven to 180C and cook the dish for 30 minutes. After this time, remove the tin foil and cook again for another 10 minutes to crisp the breadcrumbs