You all abviously (with the exception of the velveeta person) can make mac and cheese. I truly think the right cheese can make all the difference if you want magic.
I have also had the cheese separate when I made very large batches. I forgot what the chef I asked told me, and it is too late to wake him up. Now I make smaller batches.
I think gruyere adds something, as one of the 3-4 cheeses. I think my standard recipe (out of my head) is 1 c raw mac to 1 c white sauce to 8 oz. cheese. Yes on the onions (grated very fine, the mustard, the tabasco, sometimes the crumb topping, and, in my house with no kids, baby peas.
If I spare some Couger Gold for it (hard to do) I choose my recipe carefully and plan ahead.
My favorite tuna hot dish has cheese, green pepper, peas, pimento, celery, and cream of celery soup. It is truly wonderful, no matter how it sounds.