Begin with the slaw as its flavour will improve after 1 hour in the bowl. Stir together the cabbage, spring onions (scallions), most of the coriander (cilantro) (save some to garnish) and half the sugar. Add the zest and juice of 1 lime and season generously with salt and pepper. Set aside at room temperature.
Sprinkle the cornflour (cornstarch) onto a large plate and stir in the cumin, cayenne and the remaining sugar. Season well with salt and pepper. Cut the fish fillets into chunky strips 3–4cm (1 1/4–11 /2in) wide. Toss the fish pieces in the seasoned flour to coat.
It’s best to cook fish in a fish cage for easy turning and to minimize sticking. Lay the fish pieces snugly together on the fish cage ready for the grill. Fire up the barbecue ready for direct grilling For cooking fish it’s good to have a really hot, clean grill surface so give the bars a good scrub with a wire brush when they’re hot, even if you are using a fish cage.
While the grill is heating, make the lime cream. In a small bowl, stir together the sour cream, mayonnaise, garlic and the zest and juice of the lime. Season with salt and pepper.
Divide the tortillas into two piles of 6, wrap each in foil and set onto the grill bars to warm up for 2 minutes, turning them halfway. Set aside to keep warm while you cook the fish
Generously drizzle olive oil over both sides of the fish and lay the cage onto the grill bars over the fire. Lower the lid and cook for 2–3 minutes on each side until lightly charred and cooked through.
To serve, spoon some slaw into the centre of each warmed tortilla and top with a few slices of avocado. Use forks to flake the fish and add a few large chunks to each tortilla. Drizzle with some of the lime cream and scatter over the remaining coriander. Add chilli sauce if you like. Fold up the sides and tuck in quick!