3 bunches collard greens
1 tablespoon olive oil
1/2 onion, thinly sliced
2 cups low-sodium chicken broth
2 tablespoons soy sauce
1/2 teaspoon freshly ground black pepper
1/4 teaspoon garlic powder
1/4 teaspoon salt
1 tablespoon apple cider vinegar
Hot pepper vinegar, for serving, optional