Put a large pan of salted water on to boil. Peel the potatoes and chop them into chunks around 2cm across. When the water is boiling, add them to the pan. Simmer for xx mins till the potatoes are tender when pressed with a fork.
While the potatoes simmer, trim the root and top 3cm off the leek. Slice a cross into the top of the leek and rinse out any grit. Finely slice it. Halve the cabbage. Finely shred one half, discarding any really thick cores (see our tip below on what to do with the remaining cabbage half).
Warm a medium pan over a medium-low heat for 2 mins. Add 1/2 tbsp olive oil and the leek. Season with salt and pepper. Cook for xx mins, stirring occasionally, till the leek is soft and a little browned.
While the leek cooks, heat your grill to high. Line a baking tray with foil.
Add the cabbage to the leek. Cook for xx more mins, stirring occasionally, till the cabbage is soft and wilted. Set aside once it's done.
While the cabbage cooks, roughly chop the parsley leaves and stalks. Put to one side. Finely chop the chives and put them aside separately.
Drain the potatoes and let them steak dry in the colander for a few mins. meanwhile, lay the hake fillets on the baking tray. Slide under the grill and cook for xx mins till the fish is white and opaque. It will flake easily when pressed with a fork.
Pour the crème fraîche into a small pan. Add the mustard and most of the chives. Warm over a lowish heat for xx mins, stirring occasionally till it's steaming hot. Take off the heat and set aside, lid on.
Tip the potatoes into the pan with the leeks and cabbage. Add the butter and parsley and mash together over a low heat till they are combined and smooth. Season with a pinch of salt and pepper.
Divide the mash between two warm plates. Put the mustard sauce back on the heat and add a squeeze of lemon juice. Warm through for 2 mins. Place a hake fillet on top of the mash and then spoon over the mustard sauce. Garnish with the remaining chives to serve.