Trim off the stems and remove any limp leaves from the Brussel Sprouts. Blanch the sprouts in boiling water to cover for 5 minutes. Drain and rinse under cold water to stop the cooking. Heat a large frying pan and add the olive oil, garlic and onion. Saute a few minutes until the onion just becomes tender. Add the blanched drained Brussel Sprouts. Saute a few minutes until the Brussel Sprouts are cooked to your liking. Add the vinegar and toss so that all the sprouts are coated with the vinegar. Add the butter and salt and pepper to taste and toss together again.