Combine marinade ingredients.

Pour into a baking dish large enough to hold the eight onion halves. Place onions cut side down in the marinade and marinate overnight in the refrigerator.

Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius). Flip onions over and top each half with half a tablespoon of margarine.

Roast for one hour or until golden brown. Baste with marinade a couple of times during the baking.