Preheat oven to 425 degrees F.
Line a large baking sheet with aluminum foil or parchment paper for easy cleanup.
If Brussels sprouts are still attached to the large stalk, “pop” them off with your thumb.
Rinse Brussels sprouts well and shake or pat with towel to remove extra water. Cut off tough stems and slice sprouts in half lengthwise. Quarter extra-large sprouts so that all cut sprouts are roughly the same size for even cooking.
Place cut sprouts in a medium bowl, including any loose leaves, which will get extra crispy when roasted. If using optional sliced onion, add now. Add 1 1/2 to 2 TBS olive oil, salt and pepper to the bowl. Toss well to combine.
Transfer bowl contents to the baking sheet and spread out in a single layer. Cook for 10 to 15 minutes and stir. Then cook for another 10 to 15 minutes, checking frequently. When sprouts are nearly done (turning nicely brown and caramelized), drizzle with up to 1 TBS balsamic vinegar and return to oven for 5 minutes more minutes. Finished Brussels sprouts should be charred on the outside and tender inside with a nutty flavor.
Serve warm and enjoy!