Mac-and-cheese just became a weeknight possibility thanks to this easy no-roux recipe. Traditionally, mac-and-cheese begins with a béchamel sauce that is thickened by a butter and flour roux. In this version, we skip the roux all-together and use good ol’ American cheese. American has an ingredient in it called sodium citrate that binds it together and gives it its supreme meltability. Here, this magical compound gives our sauce a thick Velveeta-like creaminess, but with the flavor of more complex cheeses like cheddar and gruyere.
Preheat the oven to 400° and butter a 3½-quart baking dish.
In a large pot of boiling salted water, cook the pasta until barely al dente, about 8 minutes. You want to slightly undercook your pasta so it does not fall apart in the oven. Drain, reserving 1 cup of pasta water. Return the pasta to the pot.
In a large saucepan, bring the half-and-half to a bare simmer over moderate heat. Do NOT let it boil. Whisk in the American cheese, mixed shredded cheeses, mustard and 2 tablespoons of the butter until smooth and the cheese is melted. Season with salt and pepper. Pour the cheese sauce over the pasta and gently stir to coat. Add pasta water ¼ cup at a time if the sauce is too thick. Scrape the mac-and-cheese into the prepared baking dish.
In a medium microwave-safe bowl, microwave the remaining 2 tablespoons of butter on HIGH until just melted, 15 to 30 seconds. Add the breadcrumbs, parmesan and thyme and toss to coat. Season with salt and pepper. Scatter the breadcrumbs all over the top of the mac-and-cheese and sprinkle with flaky sea salt. Bake for 20 to 25 minutes, or until lightly golden and bubbling. Let cool slightly, then serve right away.