Fill a large pot with oil halfway up the pot. Heat oil to 180°C. Preheat grill.
In a medium bowl, mix together 150g of flour, cumin, cayenne and salt. Slowly whisk in beer until thick, but not runny. Dredge fish in remaining flour first, then dunk in the batter. Fry in batches until golden brown, about five minutes. Drain on paper-towel lined plate.
3) Meanwhile, heat tortillas on grill.
Serve fish in tortillas topped with quick-pickled onion and cucumber.
For the quick-pickled onion and cucumber:
In a medium saucepan, bring to a boil the vinegar, sugar, water and salt. Add onions and cucumber and continue to cook a few minutes more. Remove from heat, add chilli, and transfer to a bowl. Let cool slightly then chill covered, for at least one hour.
Toss in the tomato and coriander, five minutes before serving.