Preheat oven to 425 degrees F (220 degrees C). Bring a large pot of water to a boil. Add Kosher Salt (1 tablespoon). Boil Pasta (0.75 pound) for 2 minutes less than the box's suggested al dente time. Drain. Do not rinse. Set aside.
Meanwhile, melt Unsalted Butter (4 tablespoon) over medium-high heat in a medium saucepan. Add All-Purpose Flour (1/4 cup), whisking constantly for about a minute. Add 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. Kosher Salt (1 1/4 teaspoon) and Freshly Ground Black Pepper (to taste). Simmer until mixture begins to thicken, about 20 minutes.
Chop the Kale (8 ounce) into small pieces. In a large mixing bowl, toss pasta with thickened bechamel and Parmesan Cheese (1 1/2 cup), then fold in chopped kale.
Line a rimmed sheet pan with a sheet of parchment paper. Spread macaroni mixture over top. Distribute Fontina Cheese (6 ounce) over the top.
Bake for 25 to 30 minutes, checking after 10 minutes or so. The gratin should be bubbling and golden at the edges. Let sit 10 minutes before serving.