Grind rock salt and both types of dried chillies into a fine powder using a coffee grinder.
Toss cucumber wedges with lime juice, zest, and 5ml of the spice powder; marinate for five minutes.
Combine polenta and breadcrumbs in one bowl and beat the egg in another.
Season the mackerel fillets with salt and black pepper.
Coat each fillet in the beaten egg and then dredge thoroughly in the polenta mixture.
Fry the fillets in hot oil for 4 minutes per side until golden and opaque.
Add 25ml of tequila to the marinated cucumbers.
Plate the cucumbers, sprinkle with remaining spices, and top with a mackerel fillet.