What you'll need
1Preheat the oven to 375°F.
2In the baking dish and using a baking spatula, mix the macaroni with the vegetable oil until the macaroni is well coated.
3Pour the warm water onto the macaroni. Cover tightly with a lid or foil. Bake for 30 minutes. Get help taking it out of the oven. Keep covered for at least another 15 minutes. The macaroni softens some more and the baking dish cools off a bit.
4Meanwhile, make the sauce. In a large microwave-safe bowl, melt the butter in the microwave at 50% power (about 1 minute). Use a whisk to mix in the flour until smooth. Then mix in the mustard and salt. Gradually whisk in the milk until smooth.
5Heat the sauce in the microwave on high for 1 minute (or longer, but only 1 minute at a time so it doesn’t foam over), until the sauce is bubbly and thickened. Get help removing the bowl from the microwave. Cool slightly so the bowl isn’t too hot to touch.
6Stir the cheddar into the sauce. The cheese does not need to be fully melted in right now. Set aside.
7Next make the breadcrumb topping. In a large microwave-safe bowl, heat 1 Tbsp butter in the microwave at 50% power until melted (about 30 seconds). Mix in the breadcrumbs and salt. Use the back of a wooden spoon to mash the breadcrumbs into the butter to break up lumps.
8Get help uncovering the dish of macaroni (there should be some water left in the bottom of the dish). Using a baking spatula, scrape the cheese sauce onto the macaroni. Stir well. Spread the macaroni evenly in the dish. Sprinkle the breadcrumbs on top. Bake, uncovered, for about 20 minutes, or until the breadcrumbs are lightly golden.