For the pasta: Place an oven rack in the center of the oven. Preheat the oven to 350 degrees F. Butter a 13- by 9- by 2-inch glass or ceramic baking dish.
Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, 8 to 10 minutes.
In a large nonstick skillet, heat the olive oil over medium-high heat. Add the pancetta and cook until brown and crispy, about 8 minutes. Using a slotted spoon, remove the pancetta and drain on paper towels.
For the sauce: In a 2-quart saucepan, melt the butter over medium heat. Add the flour and whisk until smooth, about 2 minutes. Gradually add the milk, whisking constantly to prevent lumps. Add the lemon zest. Simmer over medium heat, whisking constantly, until the sauce thickens, about 10 minutes. Remove the pan from the heat and stir in the Parmesan, salt and pepper.
In a large bowl, combine the cheese sauce, cooked pancetta, pasta, basil and thyme. Toss until the ingredients are coated. Stir in the lemon juice. Spoon the mixture into the prepared baking dish and sprinkle with the mozzarella.
For the topping: In a small bowl, combine the breadcrumbs and Parmesan.
Sprinkle the topping over the pasta mixture. Drizzle the top with olive oil and bake until the top is golden brown, 25 minutes. Cool for 10 minutes and serve.
The topping can be drizzled with the pancetta cooking juices in place of the olive oil.