Saute the finely chopped onions in a pot and add the tomatoes.
Once everything softens, pour in 4 cups water (or more) and add the fish that's been cleaned and chopped ahead of time.
Boil about 20-25 min. and add the spices (excluding the lemon juice) and chopped garlic. Cook for another 4-5 min. and remove from the heat. Serve with lemon juice.