Preheat the oven to 350F and position the top oven rack in the middle of the oven.
Assemble the nachos:
Spread half of the tortilla chips on the cast iron skillet. I recommend using a large, 12-inch skillet for this. Top this first layer with half of the pulled pork, half of the cheese, half of the bbq sauce, and half of the chopped red onion (if using).
Make a second layer of chips over the first layer and top with the rest of the pulled pork, red onions, and cheese.
Melt 'em:
Place the skillet in the preheated oven and bake for 10 to 12 minutes until the cheese looks melty. Remove the skillet from the oven.
Serve:
Drizzle with the remaining bbq sauce, distribute the sliced jalapeños over the nachos, and top with additional fresh toppings like guacamole and sour cream.