Preheat the oven to 450 degrees F. Bring a large pot of salted water to a boil.
Meanwhile, heat the oil in a medium skillet over medium heat. Add the pancetta and cook, stirring occasionally, until browned and crispy, about 8 minutes. Transfer to a large bowl using a slotted spoon. Add the mushrooms to the pancetta drippings and cook, stirring occasionally, until golden brown, 6 to 8 minutes. Transfer to the bowl with the pancetta.
Cook the penne in the salted boiling water until very al dente, about 2 minutes less than the package directions. Drain the pasta and add it to the bowl.
Add the marinara, chicken, the fontina and half of the Parmesan to the bowl and stir well to combine. Transfer the pasta to a 3- to 4-quart baking dish and top with the mozzarella and the remaining Parmesan. Bake, uncovered, until browned, about 15 minutes. Let the pasta stand about 10 minutes before serving.
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