FROM BON APPETIT
Preheat oven to 400°. Melt 1/4 cup butter in a large saucepan over medium-high heat. Add flour; cook, whisking constantly, for 1 minute. Whisk in milk and 3 cups water. Bring to a boil, reduce heat to a simmer, and cook, whisking often, until a very thin, glossy sauce forms, about 10 minutes. Stir in 1 tablespoon salt and 1/2 teaspoon pepper. Remove sauce from heat.
Toss pasta and 1 1/2 cups cheese in a 13x9x2-inch or other shallow 3-quart baking dish. Pour sauce over (pasta should be submerged; do not stir) and cover with foil. Bake until pasta is almost tender, about 20 minutes.
Meanwhile, melt remaining 1/4 cup butter in a large skillet over medium heat. Add garlic, panko, and parsley and toss to combine. Season with salt and pepper.
Remove foil from dish. Sprinkle with remaining 1/2 cup cheese, then panko mixture. Bake until pasta is tender, edges are bubbling, and top is golden brown, about 10 minutes longer. Let sit 10 minutes before serving.