How to Make It
Preheat oven to 375°F. Heat oil in a 12-inch, high-sided skillet over medium-high. Add onions; cook, stirring occasionally, until softened, 10 to 12 minutes. Stir in radicchio, garlic, vinegar, and 1/2 teaspoon salt; cook, stirring often, until radicchio is just wilted, about 1 minute. Stir in goat cheese and cream; bring to a simmer over medium-high. Remove from heat, and set aside.
Bring 3 quarts water to a boil in a large pot over high. Stir in 3 tablespoons salt. Add pasta, and cook until just shy of al dente, 7 to 9 minutes. Drain; reserve 1 cup cooking liquid. Stir pasta, reserved 1 cup cooking liquid, pepper, nutmeg, and remaining 1 teaspoon salt into radicchio mixture. Fold in 11/2 cups fontina cheese and prosciutto; toss well.
Transfer pasta mixture to a 3-quart baking dish lightly greased with butter. Dot with remaining 1/2 cup fontina cheese; sprinkle with Parmigiano-Reggiano. Bake in preheated oven until golden brown on top and bubbling around edges, 25 to 30 minutes. Let stand 5 minutes before serving.