Preheat oven to 400° F.
In a small bowl, combine brown sugar, smoked paprika, kosher salt, chili powder, onion powder and garlic powder.
Trim the pork tenderloin of any excess fat and silverskin. Pat the pork dry, rub with 1 tablespoon of olive oil; sprinkle about 1 tablespoon of the spice rub onto the meat.
In a medium bowl, stir together Brussels sprouts, remaining 1 tablespoon of olive oil, and about 1 tablespoon of the spice rub. Set aside.
Heat a 12-inch cast iron skillet over medium heat. Add pork; cook until browned on all sides, about 4 minutes total. Remove pork from skillet.
With the skillet still over medium heat, arrange the sprouts cut-side down in the pan. Cook until lightly browned on both sides, about 6 minutes total. Return pork to the skillet with the sprouts.
Transfer the skillet to the oven and bake for 5-8 minutes, or until the pork reaches an internal temperature of 145° F. Let the pork rest for 5 minutes, then slice and serve. Season with salt and pepper, to taste.