Preheat the oven to 400 degrees F. Gently heat the oil in a cast iron skillet, then add the sprouts, cut-side down. Cook without moving until they brown nicely and develop a crust. This is where the flavor happens. Transfer the skillet to the oven and roast for 4 minutes. Remove from the oven and, using tongs, turn each sprout over carefully onto its back. Add the balsamic vinegar to deglaze, gently shaking and tossing the skillet until there is no excess vinegar in the pan. Season to taste with salt and pepper, and serve immediately.
