Heat oven to 375 degrees.
Trim bottom of brussels sprouts, and slice each in half top to bottom.
Heat oil in a cast-iron pan over medium-high heat until it shimmers.
Put in garlic, and sprinkle with salt and pepper.
Cook until sprouts, begin to brown on the bottom, and transfer to oven.
Cover the pan with heavy duty foil for the first 10 minutes so the sprouts steam and soften a bit
Roast, shaking the pan every 5 minutes, until sprouts are quite brown and tender, about 10 to 20 minutes.
Taste, and add more salt and pepper if necessary.
Stir in balsamic vinegar, and serve hot or warm.