Remove the white ribs from the centers of the collard leaves. Stack the leaves on top of each other and slice into 1-inch pieces. Thoroughly wash the greens (I soak them and continue to change the water until no dirt or sand remains). Set aside.
Heat the olive oil in a Dutch oven over medium heat. Add the onions and cook until translucent, about 5 minutes. Pour in the broth and use a wooden spoon to scrape up any brown bits. Stir in the soy sauce, pepper, garlic powder and salt. Bring to a low boil. Add the greens and reduce the heat to a very low simmer. Cover and cook, stirring occasionally, for 30 minutes. Stir in the cider vinegar and simmer an additional 15 minutes.
Serve with hot pepper vinegar if desired.