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1
Adjust oven rack to lowest level and preheat oven to 400F (200C).
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2
Do not peel the onions.
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3
Slice each onion in half crosswise.
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4
Cut two small X's near the stem or root of each onion to allow steam to escape and to help prevent the outer ring from rising.
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5
Line a jelly roll pan or cookie sheet with aluminum foil.
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6
Rub the oil evenly over the foil.
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7
Arrange the onions cut side down on the foil lined sheet.
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8
Roast for about 30 minutes or until the bottoms turn dark brown and caramelized or when tender when pierced with a small knife of skewer.
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9
Meanwhile, if desired, reduce the balsamic vinegar over medium high heat until the desired glaze thickness is achieved, usually reduced by about 1/2.
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10
Be careful of over reducing, the glaze will get thicker as it cools.
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11
Remove the onions from the oven and let rest for 5 minutes.
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12
Gently lift from the pan using a spatula, peel if desired or serve with the skins intact.
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13
Season with salt and pepper to taste and drizzle with the balsamic glaze (if using).
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14
The resulting onions are wonderful served as a side with roasts.
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15
They can also be peeled and chopped then added to grilled cheese sandwiches, salads, to enrich mashed potatoes, mixed with beans with fresh herbs for a bruschetta or served along with antipasto.
