In a shallow bowl, whisk together beer, flour and salt.
Rinse fish and pat dry.
In a large saucepan, heat 3cm oil to 170 degrees C. Using a fork, coat fish pieces in batter. Slide coated fish into hot oil in batches; adjust heat to maintain oil temperature. Fry until golden, about 2 minutes. Lift out with a slotted spoon and drain briefly on paper towels; keep warm. Repeat to fry remaining fish.
On the centre of a tortilla, place a piece of fish and cabbage. Garnish with mayonnaise, lime wedges and salsa.