Heat olive oil in large cast iron or another oven-safe skillet over medium-high heat. Add Brussels sprouts to skillet and season with salt and pepper, to taste. Cook for several minutes, occasionally stirring until the sprouts begin to turn brown.

Once browned, transfer the skillet to the preheated oven and roast for 15-20 minutes, stirring halfway through. Remove sprouts from oven once they are tender and start to turn a vibrant brown color.

While the sprouts are roasting, add balsamic vinegar and maple syrup to a small skillet and heat over medium-high heat. Bring to a boil, stirring constantly, then reduce heat to medium. Continue stirring until the vinegar mixture reduces to about 1/3 of its original volume and becomes thick and syrupy. (Do not overcook or it will get too hard and sticky).