Sift the flour, salt, and baking powder into a roomy bowl; then whisk in the ice-cold beer to make a thick and smooth batter.
Preheat the oven to 120°C.
Add the oil to a pan wide enough for the mackerel fillets and heat to 185°C.
Dip a mackerel fillet into the batter, lower it into the oil and fry for 4-5 minutes until golden and crisp.
Drain the fillet on kitchen paper and keep it warm in the oven when cooking the rest.
When all the fillets are ready, slit them along their length.
Sprinkle them with salt, squeeze the lime wedges over them, and then squirt with mayonnaise and chilli sauce.
