This came up in a search for sheet pan meals — I was wondering how pasta would work there, but instead it is a way to cook Mac and Cheese so you get more of the delicious crispy bits on top; its not really a one pan sheet pan meal.  Ah well, still sounds like a great idea =)

3/4 pound elbow macaroni
8 tablespoons unsalted butter, divided
1/4 cup flour
2 cups whole milk
2 cups water
1 1/4 teaspoons kosher salt
Freshly ground black pepper to taste
6 oz fresh bread to be pulsed into crumbs (3 cups once pulsed)
parsley, a medium to large bunch
2 to 3 cloves garlic
1/4 teaspoon kosher salt
1 1/2 cups (4 ounces) Parmigiano Reggiano, grated
1 cup (8 oz) Fresh Mozzarella, diced into small cubes

Preheat oven to 425°F. Bring a large pot of water to a boil. Add a tablespoon of kosher salt. Boil macaroni for about 5 minutes or for 2 minutes less than the box’s suggested al dente time. (For example, my macaroni box said ‘al dente perfection’ in 7 to 9 minutes. I boiled mine for 5.) Drain. Do not rinse. Set aside. In the same large pot, melt 4 tablespoons of butter over medium-high heat. Add flour, whisking constantly for about a minute. Add milk and water, whisking to remove any of the flour-butter mixture from bottom of pan. Bring to boil, then reduce heat and gently simmer. Add 1 1/4 teaspoons kosher salt and pepper to taste. Simmer until mixture begins to thicken, about 20 minutes. Note: the mixture will not get super thick, but it will coat the back of a spoon, and if you run your finger through the coating, the trail should stay — see photo. Remove from the heat. Meanwhile, melt remaining 4 tablespoons of butter stovetop or in microwave. Pulse bread in food processor to make crumbs. Measure 3 cups and place in a mixing bowl. Add parsley (it’s ok if the tender stems are included — use the top 3/4 or so of the bunch) and garlic to food processor. Pulse until fine, then add to bowl with breadcrumbs. Season with 1/4 teaspoon kosher salt. Pour butter over top and mix with spatula until combined. In a large mixing bowl, toss macaroni with grated Parmigiano Reggiano — it’s ok if the noodles are sticking together; they will separate once the béchamel is poured over top. Line a jelly roll pan with a sheet of parchment paper. Spread macaroni mixture over top. Distribute mozzarella cubes over top. Scatter breadcrumb mixture evenly over top. Bake for 25 to 30 minutes, or until top is golden and the macaroni is bubbling. Let sit 10 minutes before serving.