Heat the butter in a medium-sized, deep-sided frying pan over a medium heat. When the butter is foaming, add the leeks and season with a half-teaspoon of salt. Fry for 3-4 minutes, or until the leeks have softened and are golden-brown.
Add the chorizo and smoked paprika and continue to fry for 4-5 minutes, stirring occasionally, until the chorizo has browned and is crisp.
Add the stock and bring the mixture to a simmer. Continue to simmer gently for a further 3-4 minutes, or until most of the liquid has evaporated.
Meanwhile, heat the oil in a deep, heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Alternatively, heat the oil in a deep-fat fryer to 140C.
Meanwhile, sprinkle the flour onto a plate. Dredge the shallot rings in the flour until completely coated.
When the oil is hot, carefully lower the shallot rings into the oil. Fry for 2-3 minutes, or until crisp and golden-brown. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper. Season with another half-teaspoon of the salt, then set aside and keep warm.
Meanwhile, heat one tablespoon of vegetable oil in a non-stick frying pan over a medium heat. Season the mackerel fillets all over with the remaining half-teaspoon of salt. When the oil is hot, place the seasoned mackerel fillets, skin-sides down, into the pan and fry for 3-4 minutes, or until the skin is crisp and golden-brown and more than half of the mackerel flesh has turned opaque.
Remove the pan from the heat, then turn the mackerel fillets over and allow to cook through on the residual heat for a further 1-2 minutes.
To serve, spoon the leek and chorizo mixture into the centre of each of two serving plates. Place some of the crisp shallot rings on top of each portion. Place one mackerel fillet on top of each.