Mix the cornstarch with 1 tablespoon of pepper and 1 tablespoon of salt. Coat the fish with cornstarch and shake off the excess.
Heat oil over medium heat in a non-stick pan. Fry the fish fillets one by one until they are golden brown.
For the escovitch (sauce)
Heat the olive oil over low heat. Add the onions and cook and stir until the onions become translucent. Add the tomatoes and pepper and cook and stir for a minute. Add the tomato sauce and water. Cover and simmer until the tomatoes and pepper are cooked through.
Season with salt to taste. Add the basil and mix. Carefully place the meat on the sauce, and turn until it is coated with the sauce on all sides.
For a healthier version (non-fried), skip steps 1 to 5 and add the fish raw to the sauce. Simmer a little longer until the fish is cooked through. Each method produces a different dish, but both are delicious.