Fill a pan with water. Sprinkle in a good pinch of salt and bring to the boil. Scrub the potatoes and chop them into bite-sized chunks. When the water is boiling, add the potatoes and simmer for 15-20 mins till tender.
Meanwhile, peel the celeriac, slicing off the roots on a diagonal, and coarsely grate it into a bowl. Squeeze in the juice from half the lemon. Peel and finely grate in the garlic clove. Spoon in the yogurt and sprinkle in a little salt and pepper. Peel the horseradish and finely grate a little into the yogurt – don’t use it all to start with. Stir everything really well and have a taste, adding more horseradish if you like it hotter. Set aside.
Finely chop the flat leaf parsley and set aside. Separate the sprout top leaves and trim away any woody stalks. Shred the leaves.
Set your grill to high. Line a baking tray with foil and arrange the trout fillets on it, skin-side up. Rub 1 tbsp oil into the fish along with a pinch of salt and pepper. Set aside.
Pour 1 tbsp oil into a frying pan and bring to a medium heat. When hot, add the sprout tops with a pinch of salt and pepper. Fry for 1-2 mins, stirring now and then. Add 2 tbsp cold water and allow to steam for a further 2 mins, till just tender.
While the sprout tops cook, slide the trout under the grill. Cook for 3-4 mins, till the fish is just cooked through and the skin is crispy.
The potatoes should be cooked by now. Drain them and allow to steam dry for 1 min. Add the butter, parsley and a good pinch of salt and pepper to them. Stir well to coat all of the potatoes.
Spoon the potatoes, sprout tops and celeriac remoulade onto each plate and top with a trout fillet. Serve with wedges of the leftover lemon half and an extra grating of horseradish on top.