How to Make It
Preheat the oven to 375°. In a large, deep skillet, melt the butter in the oil. Add the fennel and onion and cook over moderate heat, stirring occasionally, until lightly browned, about 15 minutes; add water as needed to keep the vegetables from scorching. Add the radicchio and cook until very soft, about 10 minutes, adding water as needed. Scrape the vegetables into a bowl and let cool.
Meanwhile, in a large pot of salted boiling water, cook the pasta until al dente. Drain and cool under running water. Pat the shells dry.
Fold the ricotta, 1 cup of the Fontina and the parsley into the vegetables and season with salt and pepper. Stir in the eggs.
In another bowl, mix the marinara sauce with the heavy cream. Pour 1 1/2 cups into a 9-by-13-inch baking dish. Stuff each shell with a slightly rounded tablespoon of the filling and nestle the shells in the sauce as close together as possible. Drizzle 1 cup of the remaining sauce on top and sprinkle with the remaining 1/2 cup of Fontina.
Bake the shells for about 40 minutes, until golden. Let rest for 15 minutes. Warm the remaining sauce and serve on the side.