Put a large pan of salted water on to boil. Peel and chop the potatoes into bite-size chunks. Trim the roots and top 3cm off the leek. Halve it and rinse out any grit, then finely slice the leek. Trim the stalky bottoms off the Kalettes® and halve them. The water in the pan should be boiling by now. Add the potatoes, cover and simmer for 20 mins till the potatoes are tender.
While the potatoes simmer, set your oven to its lowest heat. Set a large frying pan on a medium heat. Add ½ tbsp oil and the leek. Season with a pinch of salt and pepper. Fry, stirring often, for 8 mins till the leek is soft and golden. Scoop into a heatproof dish or bowl and pop in the oven to keep warm.
While the leek fries, roughly chop the parsley. Finely grate the zest from the lemon. Juice 1 half. Set the zest, juice and remaining lemon half aside.
When the leeks are in the oven, put the frying pan back on the hob. Add the Kalettes®, season and stir fry for 4–5 mins till dark green and lightly charred. Scoop into another heatproof bowl and pop in the oven to keep warm.
Check the spuds – they should be tender by now. Drain and tip them back into the pan. Pour in 100ml of the milk and add a good pinch of salt and pepper. Use a masher to crush the potatoes and smoothly mash them. Stir in the cooked leeks. Pop a lid on the pan and set aside to keep warm.
Sprinkle 1 tbsp flour onto a plate and lightly season it. Dip the hake fillets in the flour to just coat both sides of the fish. Put the frying pan back on the heat. Add ¼ of the butter to the pan and set it on a medium heat. When the butter has melted, add the hake fillets, skin-side down. Fry for 4 mins then carefully flip and fry for another 3-4 mins till the fillets are opaque white and flake easily when pressed with a fork.
Transfer the hake fillets to a couple of warm serving plates. Turn the heat on underneath the mashed potato pan – keep the heat low, to just warm the potatoes through.
Add the remaining butter to the frying pan. When it has melted, swirl in the chopped parsley, lemon zest and lemon juice. Bubble for 30 secs. Take off the heat.
Scoop the mash onto the plates with the hake. Add the Kalettes®. Pour over the butter and garnish with the last of the parsley, serving the remaining lemon, sliced into wedges, on the side.