Combine the first 7 ingredients (the marinade). Rinse the well. If you're using a large fillet, cut it into serving pieces.
In a large glass bowl, layer about half the Onion slices. Pour some marinade over them. Then add the and the rest of the onions and marinade. Cover and refrigerate for at least 2 hours, but preferably overnight or all day.
When you're ready to cook, lift the fillets out of the marinade and set them aside. Pour the marinade off the onions and set the marinade aside. Cover the cubed potatoes with cool, salted water, bring them to a boil, and then simmer for a few minutes until they are just barely tender. Drain any excess liquid.
Meanwhile, in a heavy, nonreactive skillet, gently sauté the onions in the peanut oil for about 20 minutes, until lightly browned. Add the red bell pepper, if used, for the last 5 minutes of sautéing. Combine the sautéed onions and bell pepper with drained sweet potatoes and marinade and simmer for about 20 minutes.
While the vegetables simmer, briefly grill, broil, or sauté the until lightly browned on both sides. Add the to the simmering vegetables and continue to simmer for 15 minutes or until the is just cooked through. salt to taste. Serve in wide, shallow bowls on rice or steamed millet.