1. To make the guacamole, mix the avocado, tomato, chilli, lime juice, onion, garlic, coriander and olive oil together.
2. Cut the flathead fillets into 20 portions and preheat the oven to 160°C (315°F/Gas 2–3).
3. Put the flour in one shallow bowl, the egg in another and the cornflake crumbs in a third. Lightly season the fish with some sea salt, then dust lightly with flour, coat in the egg and then the cornflake crumbs, patting the crumbs on firmly.
4. Heat the grapeseed oil in a wok or deep saucepan to 160°C (315°F). Fry the fish in batches for 30–45 seconds until golden and crispy, then turn over and cook for a further 30 seconds or until cooked through. Drain on kitchen paper.
5. Warm the tortillas on a baking tray in the oven for 3–4 minutes, then fill the tortillas with the purple cabbage, guacamole, fish pieces, sour cream and chilli sauce. Squeeze a bit of lemon juice over the top, then put a skewer through the sides to help hold it together. Serve immediately.