Set your oven to its lowest temperature. Fill your kettle and boil it, then pour the hot water into a large pan. Pop on a lid and set on a high heat to come to the boil. Scrub the potatoes and chop them into bite-sized pieces. When the water is boiling, add the potatoes. Simmer for 12-15 mins till the potatoes are tender when pierced with a fork.

While the potatoes cook, set your oven to its lowest setting. Trim the roots and top 3cm off the leek. Halve it and rinse out any grit. Finely slice the leek. Trim the stalks away from the sprout tops. Finely slice the leaves.

Warm a large frying pan over a high heat. Add ½ tbsp oil, the leek and sliced sprout tops. Season with a pinch of salt and pepper and cook, stirring often, for 4-5 mins till the greens have softened and are glossy.

Stir 1 tbsp mustard and the crème fraîche into the greens and cook, stirring often, for 2-3 mins. Taste the sauce and add a pinch more salt or pepper if you think the greens need it. Divide the creamed greens between 2 warm plates and pop them into the oven to keep warm.

Wipe the frying pan clean with kitchen paper and put it back on a high heat. Add ½ tbsp oil. Carefully lower in the haddock fillets and fry for 2-3 mins till golden brown underneath. Carefully turn them over and fry for a further 2-3 mins till the fillets are white and flake easily when press with a fork. Transfer the haddock fillets to the warm plates and pop them back into the oven.

Check the potatoes – they should be tender and cooked through by now. Drain them, shake dry and tip back into the pan. Add the butter and season with a pinch of salt and pepper. Gently swirl and shake the pan to coat the potatoes in the butter. Add them to the plates with the haddock and creamed greens and serve straight away.