Preheat the oven to 350 degrees.
Use leftover pulled pork from your last great BBQ or slow roast a pulled pork in your slow cooker or heavy duty roasting pan in the oven. For Stubb's Slow Cooker Pulled Pork recipe, see our Grilling Tips videos. Don't sauce the pork if you haven't already. Shred the pork into about 2' long pieces.
Cook the pasta in a large pot of well-salted water according to package directions until just barely al dente. Drain and set aside.
Melt butter in a large 8-quart saucepan over medium heat. Add onion, reduce heat to low and cook, stirring frequently until translucent and soft.
Whisk in flour and cook for 30 seconds. Whisk in milk slowly. Raise heat to medium high and whisk constantly until the mixture begins to thicken and bubble.
Whisk in Stubb's Sweet Heat Bar-B-Q Sauce and 1/2 teaspoon each salt and pepper. Bring mixture to a low simmer, whisking constantly.
Reduce the heat to low and use a wooden spoon to stir in all cheese except Pepper Jack until melted. Stir in the reserved pork and pasta until well coated. Pour the mixture into a 12' cast iron skillet.
In a small bowl, mix the panko, Pepper Jack, and olive oil. Sprinkle evenly over the mixture in the skillet.
Bake until the topping is browned and the cheese sauce is bubbling through the topping and around the edges of the skillet, around 40-45 minutes. If the topping begins to brown too deeply, tent loosely with foil. Let the dish rest for at least 15 minutes. Sprinkle with chives and serve.