Season fish with salt and pepper. Dust with flour and fry in hot olive oil until golden brown on all sides. Transfer fish to a side dish. Add onion and garlic to the skillet and cook for a few minutes. Add tomato sauce, harissa, bay leaf and 1/2 cup water. Cook for 10 minutes. Add olives and fish fillets and continue cooking, uncovered, until the fish is tender and the sauce thick. Add lemon juice to taste. Discard the bay leaf. Serve, sprinkled with parsley.