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. Stirin 1 tablespoon salt and 1/2 teaspoon pepper.
Removesauce from heat.
Toss pasta and 1 1/2 cups cheese in a13x9x2" or other shallow 3-quart baking dish.
Pour sauce over (pasta should be submerged;do not stir) and cover with foil.
Bake untilpasta is almost tender, about 20 minutes.
Meanwhile, melt remaining 1/4 cup butterin a large skillet over medium heat.
Addgarlic, panko, and parsley and toss tocombine. Season with salt and pepper.
Remove foil from dish.
Sprinkle withremaining 1/2 cup cheese, then pankomixture.
Bake until pasta is tender, edges arebubbling, and top is golden brown, about10 minutes longer.