Ingredients

Step by step

1. Tip the cannellini beans into a large bowl and use a potato masher to crush to a coarse mash. Add the spring onions, horseradish sauce, the zest of the lemon and half the juice, the tarragon, egg yolk, salt and plenty of black pepper. Mix well to combine, then flake in the skinned mackerel (keeping an eye out for any larger stray bones). Stir until just combined. With floured hands, shape into 8 patties (each about 7cm in diameter); put on a lined baking tray (or trays) and chill for at least 1 hour. Cut the remaining lemon half into wedges for serving, if you like.
2. Meanwhile, for the mayo, put the egg yolk, mustard, vinegar and salt in the small bowl of a food processor and pulse briefly to mix. With the machine running, very slowly trickle the oil through the funnel until you have used it all and have a thick emulsion. Add the watercress in two batches and blend again until combined. Check the seasoning to taste, then spoon into a bowl, cover and set aside in the fridge.
3. Preheat the oven to 190°C, fan 170°C, gas 5. For best results, whiz the breadcrumbs in a food processor to a finer consistency, then tip into a bowl. Put the flour and beaten egg in separate bowls. Season the flour well with salt and black pepper. Coat each fishcake first in the flour, then in the egg, and lastly in the breadcrumbs. Try to use one hand for the dry ingredients and one for the beaten egg. Return to the lined tray(s).
4. Over a medium-high heat, heat enough oil in a wide saucepan or deep sauté pan to come 1-1.5cm up the sides. Fry the fishcakes in two batches for 2-3 minutes on each side, or until deep golden and crispy. Drain on kitchen paper and return to the tray(s). Bake for 8 minutes to finish heating through. Arrange the fishcakes on serving plates with the watercress mayo, extra watercress and lemon wedges, if using. Store any leftover watercress mayo in the fridge for up to 3 days.