Check your avocado is ripe before starting this recipe (see cooking tip). Thinly slice the onion and cabbage with a sharp knife or mandoline. Combine 1 tbs red wine vinegar and 1 tsp sugar in a large bowl. Add the onion and cabbage and toss to coat. Set aside to pickle, tossing occasionally, until needed.
Step 2 — Crumb fish
Meanwhile, whisk 1 egg in a bowl. Put 2 tbs flour in a shallow dish. Combine the breadcrumbs and barbecue seasoning in a separate shallow dish. Cut the fish into 2cm-thick strips. Dust the fish with the flour, shake off the excess, then dip in the egg and coat in the seasoned breadcrumbs.
Step 3 — Cook fish
Heat 1cm oil in a medium frypan over medium heat. Cook the fish for 3½-4 mins each side until golden and cooked through. Remove from the pan and drain on paper towel. Season with salt and pepper.
Step 4 — Prep avocado
While the fish is cooking, cut the avocado into thin wedges.
Step 5 — Warm tortillas
Put the tortillas on a plate, slightly overlapping, and microwave for 30 secs or until warmed through. (If you don't have a microwave, heat a large frypan over medium-high heat. Warm the tortillas, in batches, for 30 secs each side or until heated through.)
Step 6 — Toss slaw and serve
Add the sunflower seeds to the pickled slaw and toss to combine. Divide the tortillas among plates and fill with the slaw, fish and avocado. Drizzle with the aioli to serve.