Heat oven to 400 degrees. Trim bottom of brussels sprouts, and slice each in half top to bottom. Heat oil in cast-iron pan over medium-high heat until it shimmers; put sprouts cut side down in one layer in pan. Put in garlic, and sprinkle with salt and pepper.

Cook, undisturbed, until sprouts begin to brown on bottom, and transfer to oven. Roast, shaking 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.