Preheat oven to 375°F and cook pasta according to package directions until al dente, drain and set aside.
In a large oven-safe skillet, cook pancetta over medium heat until crispy, about 6-8 minutes, then remove and set aside.
In the same skillet, melt butter and whisk in flour, cooking for 1-2 minutes.
Gradually add warm milk and cream, whisking constantly until sauce thickens, about 5 minutes.
Remove from heat and stir in Gruyère and fontina cheese until completely melted and smooth, season with salt and pepper.
Add cooked pasta and half the pancetta to the cheese sauce, stirring to combine.
Mix panko breadcrumbs with Parmesan cheese and sprinkle over the top, then add remaining pancetta.
Bake for 15-20 minutes until golden and bubbly, then drizzle with truffle oil and garnish with fresh parsley before serving.