1. Preheat oven to 350. Cook pasta in boiling salted water according to package instructions. Drain pasta and set aside.
2. Melt 1 T. of butter in a large, deep skillet. Add pancetta and let it crisp for 6-8 minutes, until it has the texture of almost cooked bacon. Then, add the onion and stir gently until it is soft and slightly golden. Add the garlic and red pepper, stir for 2 minutes, making sure it is evenly distributed.
3. To the pancetta mixture add 2 T. butter and allow it to melt. Add flour and stir for a minute, allowing it to thicken. Then, working quickly, whisk in milk. Be sure to keep whisking to make the roux and get out any lumps in the sauce. Stir for about five minutes, or until the milk starts to thicken and can coat the back of a spoon.
4. Remove the sauce from the heat and add the cheddar, parmesan and mascarpone. Stir well to make sure all of the cheeses are incorporated. If sauce it too thick, add more milk by the ¼ cupfuls to loosen it up. Taste the sauce and add salt and pepper as needed.
5. Gently, pour the pasta into the prepared sauce and stir to make sure it is all mixed together. Then, pour it into a 9×13 baking dish.
7. To prepare the breadcrumbs, melt 3 T. butter in a shallow pan and add panko. Swirl the panko and butter gently in the skillet with a wooden spoon, toasting panko until just golden. Remove from heat. Spread toasted panko evenly over top of the casserole.
8. Bake for 30 minutes, or until heated through. Can be made ahead of time and re-heated.