Cut ends off onions, leaving stem & core intact. Quarter onions through core & peel away skin.
Place onions in a large resealable bag with dressing, tarragon & garlic. Marinate in fridge for 1 to 4 hours.
Remove from fridge & bring to room temp.
Preheat oven to 400 degrees. Place onions cut side up in lightly greased baking pan. Pour marinade over top. Season with black pepper. Roast for about 25 minutes.