There are two camps when it comes to macaroni and cheese. Most people make a sort of bechamel sauce over the stovetop, add cheese, then toss the cheesy bechamel with cooked noodles. The minority tosses cooked pasta with ingredients separately, letting them come together gradually over the course of several add ingredients/stir/bake cycles. I am a lemming. I went the stovetop sauce route.
The benriner actually came in quite handy to grate 1/2 onion for the sauce.
I used the pot in which I cooked the penne to melt 1/4 c. butter (for the mathematically challenged like me, that's 4 T. or 1/2 a stick) and cook the grated onion until soft, about 5 minutes. Sprinkle 1/4 cup flour over the onions and butter and cook for an additional 5 minutes. Whisk in 4 c. milk (I had to use nonfat because that's all my Mom had in the fridge), bring to a boil, then reduce heat to simmer.
While it's simmering, add 1 T. hot sauce (I almost had to use sriracha, but thankfully, I found some Tabasco), 1 t. salt, 1/2 t. black pepper, and 5 mustard packets. Of course, if you actually have dry mustard, use 1 1/2 t. of that, but then I can't guarantee results like mine, since I used what I had.
Remove pot from heat and temper 2 large eggs. Okay, so I'm going to admit right now that on the first attempt, I didn't temper my eggs very well. In fact, I pretty much made a small bowl of creamy egg drop soup, which I dumped in the sink. On the second attempt at tempering, it was better, but the stupid eggs still made the sauce grainy. In the sink.
I successfully tempered 2 eggs on the third try, and dumped the whole thing into the pot of sauce. Add 5 cups of shredded cheese and stir until it's all melted. Dump cooked pasta into sauce, stir, then pour it all into a buttered 2 quart casserole. There will be a tiny bit leftover, but hey, what the heck are you supposed to eat for lunch if you don't have anything to taste on the way to dinner?
The casserole goes in a preheated 350 oven for 30 minutes.