Many (most?) mac and cheese recipes involve making a mornay sauce, but they rarely call it that. The slightly more informed recipes will actually call it a béchamel sauce and then have you add cheese.
I really hate that when I'm reading a recipe, if it just said "make a mornay sauce with 4 oz cheddar and 4 oz gruyere" you'd cut out 3 steps from the recipe. It's loop unrolling for recipes.
There are plenty of recipes that are made for people that do know the basics, or where the basics are literally factored out separately in the same cookbook. The latter is more common in large cookbooks with fairly deep coverage, the former in ones targetting a more specialized, experienced audience.
I really hate that when I'm reading a recipe, if it just said "make a mornay sauce with 4 oz cheddar and 4 oz gruyere" you'd cut out 3 steps from the recipe. It's loop unrolling for recipes.