Cut ends off onions, leaving stem and core intact. Quarter onions through core and peel away skin.
Place onions in a large resealable bag with balsamic dressing, tarragon, and garlic. Squeeze out air and seal bag. Marinate in refrigerator for 1 to 4 hours, turning occasionally.
Remove onions from refrigerator 30 minutes before roasting and bring to room temperature.
Preheat oven to 400 degrees F. Lightly spray a baking pan with olive oil cooking spray.
Place onions cut side up in prepared baking pan and pour marinade over top. Season with freshly ground black pepper.
Roast in preheated oven for 25 minutes or until fork tender. (If onions begin to brown too quickly, cover loosely with foil.)