The recipe I created for One-Pot Mac and Cheese in my book Cooking Slow yields great results and the method couldn't be friendlier, but I started thinking it could be better with a tweak or two.
My problem is that the macaroni tends to sink while the cheese tends to rise, creating a thin layer of macaroni-free cheesy custard at the top of the casserole. Giving it a stir near the end of baking would solve the blemish, but that would break up the crust on top and require more work, something I considered verboten for a recipe that strove for ease.
Well, I tried it, and for the minimal added effort, it proved to be worthwhile. This gave me the permission I needed to introduce a few other improvements. First, I took 30 minutes to pre-hydrate the macaroni so that it absorbed less milk during baking and gave the casserole a creamier finish. Plus, I toasted the bread crumbs, so that I could add them after the stirring step to create a crunchy crust that required no further browning.
Combine macaroni and water in a large bowl. Stir to moisten and set aside for 30 minutes to hydrate the pasta.
Preheat the oven to 200°F/95°C.
Grease the inside of a 2-qt/2-l casserole with a teaspoon or so of the melted butter. And dust the interior with 1/3 cup of the breadcrumbs.
Mix the milk, mustard, salt and pepper in a quart-size measuring cup. And pour a 1/2 cup or so into the bottom of the prepared casserole.
Drain the macaroni and toss with the shredded cheese. Scrape into the casserole and pour the remaining liquid over top. Cover the casserole with a lid or a sheet of foil and bake until the sides are set, about 2 hours.
While the casserole is baking, mix the remaining breadcrumbs with thyme and the remaining melted butter. Heat a large skillet over medium heat, add the breadcrumb mixture and stir until toasted, about 4 minutes.
Remove the casserole from the oven. Gently mix the contents to distribute the solid and liquid parts on the casserole; top with toasted breadcrumbs and return to the oven, uncovered. Bake for another 2 to 4 hours depending on your schedule.
Store for up to 4 days, covered in the refrigerator. Reheat gently in a low oven.