Bring a large pot of water to a boil. Add Kosher Salt (1 tablespoon). Boil Elbow Macaroni (0.75 pound) for about 5 minutes, or for 2 minutes less than the box's suggested al dente time. Drain. Do not rinse. Set aside.
In the same large pot, melt Unsalted Butter (1/4 cup) over medium-high heat. Add All-Purpose Flour (1/4 cup), whisking constantly for about a minute.
Add Whole Milk (2 cup) and Water (2 cup), whisking to remove any of the flour-butter mixture from bottom of pan.
Bring to boil, then reduce heat and gently simmer. Add Kosher Salt (1 1/4 teaspoon) and Freshly Ground Black Pepper (to taste). Simmer until mixture begins to thicken, about 20 minutes. 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. Remove from heat.
Meanwhile, melt the remaining Unsalted Butter (1/4 cup) on a stovetop or in microwave. Pulse Bread (6 ounce) in food processor to make crumbs. Measure 3 cups and place in a large bowl. Add Fresh Parsley (1 bunch) and Garlic (2 clove) to food processor. Pulse until fine, then add to bowl with breadcrumbs. Season with Kosher Salt (1/4 teaspoon). Pour melted butter over top and mix with spatula until combined.
In a large bowl, toss macaroni with Parmigiano-Reggiano (1 1/2 cup) — it's ok if the noodles are sticking together; they will separate once the béchamel is poured over top. Pour the béchamel over top and stir to mix.
Line a jelly roll pan with a sheet of parchment paper. Spread macaroni mixture over top.
Distribute Fresh Mozzarella (1 cup) over the top.
Scatter breadcrumb mixture evenly over top.
Bake for 25 to 30 minutes, or until top is golden and the macaroni is bubbling.