Heat 4-6 quarts of water to a boil in a heavy stockpot. Add the onions and blanch them in the boiling water for just one minute. Drain the onions and let them cool slightly.

Using a paring knife, cut off the root end of each onion and gently peel off the outer skin.

Place the onions in a large bowl and add the olive oil, melted butter, balsamic vinegar and salt and pepper to taste. Toss the mixture to combine.

Spoon the onions into a heavy oven casserole dish and roast in the oven until the onions are tender and slightly golden and caramelized, about 40 minutes. Immediately serve the onions hot from the oven.