For a simple and ultra-creamy baked pasta, take a cue from chef Ashley Christensen of Poole’s Diner: Trade the béchamel for this much easier sauce of heavy cream simmered with plenty of nutmeg and two kinds of cheese.
Cook pasta in salted water according to package directions until barely al dente, about 10 minutes (about 3 minutes less than package directions indicate). Drain and immediately transfer to a large rimmed baking sheet; spread in a single layer. Set pasta aside. Toss together Gruyère and Parmigiano-Reggiano in a medium bowl, and set aside.
Preheat broiler to high with oven rack in middle of oven. Bring heavy cream and salt to a simmer in a large saucepan over high. Reduce heat to medium; stir in nutmeg, and simmer, stirring often, 2 minutes. Remove from heat. Stir in 4 1/4 cups of the cheese mixture, in small batches, stirring until cheese is melted after each addition. Stir in cooked pasta. Spoon pasta mixture into a 2 1/2-quart broiler-proof baking dish; top with remaining 1 cup cheese mixture.
Broil in preheated oven until cheese is melted and golden brown, 3 to 5 minutes. Let stand 10 minutes before serving. Serve with thinly sliced prosciutto.
Recipe may be prepared 1 day in advance through step 2, increasing salt from 1 1/2 teaspoons to 2 teaspoons. Cover and refrigerate; let come to room temperature before broiling.