Add the chopped tomatoes to the pot and cook until they soften and release their juices.
Cook the Fish:
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:
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:
Taste the Mpiodi and adjust the seasoning if needed. Serve hot with fufu, pounded yam, or boiled rice.