Season fish with salt and pepper, sprinkle with the juice of 1 lemon. Set aside. Peel zucchini and slice into 1/4 inch rounds. Heat the oil in a skillet and fry the rounds until brown on both sides. Drain. In the sme oil, cook the onin until soft. Add the tomato sauce, harissa, garlic, salt and pepper. Cook uncovered over medium heat, stirring for 5 minutes. Return the zucchini to the tomato sauce and continue to cook at the simmer uncovered for 5 minutes. Meanwhile heat enough oil to a depth of 1 inch in a deep skillet. Dip the fish fillets in flour, beaten egg and back in the flour again. Fry until golden in oil. Drain. Add the remaining lemon juice to the tomato sauce and serve covering the fish.