Preheat the oven to 450 degrees F. Grease a 9-by-13-inch baking dish with butter; set aside.
Bring a pot of water to a boil and salt generously. Cook the pasta according to the package directions for al dente. Drain, transfer to a large bowl and set aside.
Meanwhile, mix together the panko, oil, 1 cup of the Parmesan, 1 teaspoon salt and 1/2 teaspoon pepper in a medium bowl; set aside.
Melt the butter in a medium saucepan over medium-high heat. Add the pancetta and cook, stirring frequently, until golden and crispy, about 8 minutes. Use a slotted spoon to transfer the pancetta to the bowl with the breadcrumb mixture and stir to combine.
Heat the saucepan with the drippings over medium-high heat. Whisk in the flour and cook until smooth, about 2 minutes. Slowly add the milk and cream, whisking to avoid lumps, and bring to a simmer. Cook until thickened, about 3 minutes. Turn off the heat and whisk in the peas, remaining 2 cups Parmesan, 1 1/2 teaspoons salt and 1/2 teaspoon pepper.
Pour the sauce over the pasta and stir to combine. Transfer to the prepared baking dish and sprinkle with the breadcrumb mixture. Bake until golden and bubbly, about 10 minutes. Let sit for 5 minutes, then sprinkle with additional Parmesan before serving.