MAKE THE CRUMB TOPPING:
Melt the butter in a medium saucepan over medium heat. Add the breadcrumbs and chives, stirring to combine. Cook for 1 to 2 minutes until toasted slightly. Transfer to a small bowl and set aside.
MAKE THE PASTA AND SAUCE:
Preheat the oven to 400ºF.
Slice the top one-third off the head of garlic. Place the bottom portion on a piece of foil, drizzle it with olive oil, and season it with salt and pepper. Place the top portion back on the head of garlic and crumple the foil to create a sealed packet. Roast the garlic for 30 minutes.
Remove the roasted garlic from the oven and squeeze the cloves into a bowl. Using a fork, mashing the cloves into a paste. Set aside.
Cook the pasta in a large pot of well-salted water until al dente. Drain the pasta, reserving 1/2 cup of the cooking water. Butter a 13×9-inch shallow baking dish.
In a large, heavy saucepot (such as a Dutch oven), melt 1 stick butter over medium-low heat. Stir in the flour and reserved garlic paste. Cook the roux, stirring constantly, for 3 minutes until golden. Whisk in the milk. Bring the sauce to a boil, whisking constantly, then reduce it to a simmer and let it cook for 3 minutes. Stir in the shredded cheddar cheese, Parmesan cheese, mustard powder, salt and pepper until well combined.
Add the pasta and the reserved 1/2 cup of pasta water to the pot, stirring to combine. (It will be soupy.) Transfer the macaroni and cheese to the buttered baking dish. Sprinkle it with the reserved crumb topping and place it on a sheet tray (in the event it bubbles over while baking). Bake the macaroni and cheese for 20 to 25 minutes until it’s golden brown and bubbling.