1. Preheat the oven to 350°. Butter a 3-quart baking dish. In a heatproof bowl, soak the porcini in boiling water until softened, about 15 minutes. Rinse the porcini to dislodge any grit, then drain and chop them. Discard the soaking liquid.
2. In a large, deep skillet, melt 2 tablespoons of the butter in the olive oil. Add the porcini, pancetta and one-third of the garlic and cook over moderately high heat, stirring occasionally, until the garlic is golden and fragrant, about 3 minutes. Using a slotted spoon, transfer to a plate.
3. Add the radicchio wedges to the skillet in a slightly overlapping layer and cook over moderately high heat until wilted and just beginning to brown, about 4 minutes. Add the remaining garlic, season with salt and pepper and cook, stirring, until the radicchio is slightly caramelized, about 5 minutes longer. Stir in the porcini mixture and the sage. Transfer the mixture to a large bowl.
4. Cook the pasta in a large pot of boiling salted water until al dente. Drain very well. Meanwhile, in a medium saucepan, melt the remaining 3 tablespoons of butter. Add the flour and cook over moderately high heat, stirring, until foamy, about 2 minutes. Gradually whisk in the milk and bring to a boil; cook, whisking until thickened, about 4 minutes. Add the cream and nutmeg and season with salt and pepper. Transfer the sauce to the bowl with the radicchio.
5. Add the pasta to the bowl along with the Asiago and Fontina; toss well. Transfer the pasta to the prepared baking dish. Sprinkle with the Parmesan cheese. Bake for 20 minutes, or until heated through.
6. Preheat the broiler. Broil the pasta for 2 minutes, until the top is golden and bubbling. Let stand for 10 minutes; serve.