'Macaroni and cheese' is a dish. The dish itself was singular. If I had macaroni on one portion of my plate and cheese separately on another side, they would be plural. It is as if I was saying "The dish made from macaroni and cheese..." If I mixed corn and peas together, I would refer to it in the singular, since I know it is considered one item in the list of things I ate. "The corn and peas taste pretty good."

If you have a goldfish AND a hamster in your pants, it will become singular when the hamster has eaten the goldfish. :-) And you will probably have a very strange experience! You are formally not supposed to begin a sentence with 'and'. But it adds emphasis to break that rule, as is also the case if you begin a sentence with 'but'... so long as you only do this occasionally, to add that extra weight to what you are saying.