Elbow macaroni and cheese, jalapeño, bacon, onion, meatballs

Here's the sauce, right here.

Plus 1 cup of the pasta-water.

But it could have been milk.

Oh, plus 2 tablespoons butter.

And two beaten eggs. For body. 

Body. It's a word we cook-types use for "depth" or for "substance." 

See, believe it or not, plain pasta with cheese is superficial. As if it were made with water even though it is made with milk. It's a paradox.We're over here tasting it and thinking, what does this stuff need?  It's somewhat 2-dimensional. Insubstantial even though it has cheese. Egg and butter give it weight, dimension, substance. That's what we mean by "body." With egg and butter then the whole thing suddenly seems filled out.

Egg is the difference between plain oil mixed with vinegar as dressing, and mayonnaise. Maybe that's a bad example because the added substance is whipped up to fluff. And mayonnaise has mustard.

Your mac and cheese could be made with mayonnaise and with mustard. They're perfectly workable ingredients. Because they add body.