Scrub the potatoes (peel them if you prefer) and chop them into bite-size pieces. Tumble into a pan and fill with cold water, till the potatoes are covered by 2-3cm. Add a generous pinch of salt. Cover, bring to the boil, then simmer for 15 mins.
While the spuds come to the boil, trim the roots and top 3cm off the leeks. Slice the leeks in half lengthways and rinse under a cold tap. Thinly slice the leeks. Peel the garlic and crush or finely chop it. Grate the cheddar.
Pour 1 tbsp oil into a large, ovenproof frying pan and warm over a medium heat. Slide in the leeks and garlic with a pinch of salt and pepper. Cook for 5 mins, stirring occasionally, till softened.
Add 1 tbsp flour into the leeks and stir well. Cook for a further 2 mins, stirring frequently to stop the flour from catching on the pan.
Slide the cheddar into the pan and pour in half the pot of cream. Stir well and cook for 1-2 mins, till the cream is warmed through. Add the spinach leaves and stir till wilted.
Preheat your grill to medium-high. Nestle the haddock fillets into the pan with the creamy veg. Sprinkle over a little salt and pepper and slide under the grill. Cook for 7-8 mins, till the fish is cooked through and flakes easily when pressed with a fork. If your frying pan isn’t suitable for grilling, then transfer the creamy veg to a heatproof dish, tuck the haddock in with it and pop under the grill.
While the haddock is cooking, finely chop the parsley leaves and stalks. Pull any ragged leaves off the Brussels sprouts and halve or quarter any large ones. Check the potatoes. They should have simmered for 15 mins by now and should be almost cooked through – a skewer inserted into one will nearly slide through. Add the Brussels sprouts to the pan. Cook for a 2-3 mins, till the sprouts are just tender and the spuds are ready. Drain well and tip into a bowl.
Pour 1 tbsp oil over the potatoes and sprouts. Add the chopped parsley. Squeeze over the juice from the lemon and sprinkle over a pinch of salt and pepper. Gently stir to combine.
Serve the grilled haddock with the creamy veg and the herb and lemon potatoes and Brussels sprouts on the side.