Place the flour, spices, oregano and measurement water in a shallow dish and stir together until it resembles thick cream. Season the fish, then dip in the flour mixture until well coated.
Fill a large saucepan one-third full of oil and heat to 180–190°C (350–375°F), or until a cube of bread browns in 15 seconds. Let any excess drip away from the fish, then deep-fry in batches for 3–5 minutes until golden and crisp. Remove from the pan, drain on kitchen paper and keep warm.
Make the tartare sauce by mixing together the mayonnaise, chilli, capers and gherkin in a bowl. Add the lime rind and juice to taste, then stir in the coriander.
Toss the cabbage with a little salt and a dash of lime juice. Divide among the tortillas and add the deep-fried fish. Drizzle over the tartare sauce and serve.