Preheat oven to 400 degrees F.
Combine ¾ cup dry sherry, ¼ cup honey, 1 tablespoon apple cider vinegar, ¼ teaspoon salt, and ¼ teaspoon freshly ground black pepper, and whisk until blended.
Spray an extra-large baking dish or two 9x13-inch dishes with cooking spray and pour in the onions.
Pour the sauce over the onions and stir gently to mix.
Bake for 1 hour and 15 minutes or until golden brown, stirring occasionally.
If onions start to dry out, add a splash or two of red wine or balsamic vinegar.
