Preheat oven to 300°F (no, this is not a typo; cooking at a low temperature is important). Arrange onions in one layer in a 9 x 13- inch pan. Top with flanken. Set aside.
In a bowl, combine orange juice, sugar, dissolved potato starch, salt, wine, oil, and garlic powder. Mix well.
Pour sauce over meat. Cover pan with two layers of heavy duty foil. Make sure it is sealed very well. Bake for 41⁄2 hours.
If you prefer, you can place it in the oven at 200 degrees for about 8 hours (great way to utilize your oven while you sleep).
This meat can be prepared in advance and rewarmed. This works for me. When the meat is cool, the fat congeals and can be removed easily.