Lightly toast the cumin and caraway seeds, crush and set aside.
Preheat the oven to 220ºC/425ºF/gas 7. Place the potato on a small baking tray and bake for 45 to 60 minutes, or until soft in the middle. Slice in half, scoop the flesh into a bowl (you should have roughly 350g); the skin can be discarded (or eaten!). Roughly mash the flesh and set aside to cool. Turn the oven down to 190ºC/375ºF/gas 5.
Bring a small pan filled with plenty of water to the boil over a high heat. Add the quinoa and cook for 14 to 15 minutes, or until it’s cooked but still retains a bite. Drain and stir through the potato.
Add 3 tablespoons of oil to a medium frying pan and place over a medium–high heat.
Peel and finely chop the onion. Once the pan is hot, add the onion and a pinch of sea salt and fry for 8 minutes, or until soft and caramelised, stirring frequently.
Stir in the cumin, caraway and chilli flakes for 1 minute, then add the onion and spices to the potato and quinoa.
Flake the mackerel into 1 to 2cm pieces, pick and roughly chop the herbs and beat the egg; add it all to the mixture along with ¾ of a teaspoon of salt, the zest of 1½ lemons and plenty of freshly ground black pepper.
Mix to combine. Shape the mixture into 12 patties 6 to 7cm wide and 2 to 3cm thick, then set aside.
Add 2 tablespoons of oil to a large frying pan and place over
a medium–high heat. Once hot, add half the patties and fry for
3 to 4 minutes, or until golden-brown on both sides, carefully turning halfway through.
Remove from the pan and place on a lined baking tray. Add another tablespoon of oil to the pan and continue with the remaining patties in the same way.
Finish them off in the oven for about 8 minutes, or until cooked through. Cut the remaining lemon into wedges and serve with the hot patties along with soured cream for dipping, if you like.