Special equipment: a 12-inch cast-iron skillet
Preheat the oven to 350 degrees F.
Bring a large pot of well-salted water to a boil. Cook the spaghetti according to the package instructions for al dente. Drain well and let cool slightly.
Whisk together the tomato sauce, fontina, milk, basil, parsley, eggs, 1 cup of the Asiago, 1 teaspoon salt and 1/2 teaspoon pepper in a large bowl. Add the spaghetti and toss to coat.
Heat a 12-inch cast-iron skillet over medium-high heat and add the pancetta. Cook until golden and crispy, about 5 minutes. Transfer to the bowl with the pasta and toss to combine. Let the skillet cool for 5 minutes.
Pour the pasta mixture into the skillet and sprinkle the remaining 1 cup Asiago on top. Bake until the cheese is melted and bubbling and a knife inserted and turned in the center of the pie does not release any loose egg batter, 45 to 50 minutes.
Run a spatula underneath the pie to loosen the base. Slide it onto a serving plate and cut into wedges.
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