Prepare the Fish:

1. Clean and gut the horse mackerel, then rinse them thoroughly.

Sauté the Vegetables:

1. Heat the vegetable oil in a large pot over medium heat. Add the chopped onion and sauté until it becomes translucent.

Add the Tomatoes:

1. Add the chopped tomatoes to the pot and cook until they soften and release their juices.

Cook the Fish:

1. Add the horse mackerel to the pot and stir well to combine with the onions and tomatoes. Pour in enough water or broth to cover the fish.

Season and Simmer:

1. Add the salt, ground black pepper, ground ginger, and ground garlic (if using). Stir well to combine all the ingredients. Bring the mixture to a boil, then reduce the heat to low and let it simmer for about 15-20 minutes, or until the fish is cooked through.

Serve:

1. Taste the Mpiodi and adjust the seasoning if needed. Serve hot with fufu, pounded yam, or boiled rice.