Preheat the oven to 180C/160C Fan/Gas 4.
Place the celeriac halves on 2 separate pieces of foil, large enough to cover the celeriac. Drizzle 1 tablespoon of oil over each half, along with the thyme and ¾ teaspoon of salt (on each half) and rub everything evenly. Finish with a generous grind of pepper and then seal the celeriac in the foil. Bake for an hour, until soft. Remove from the oven, unwrap and foil and set aside for about half an hour, for the celeriac to come to room temperature.
Meanwhile, mix together the crème fraiche and horseradish, with a pinch of salt and a grind of pepper. Keep in the fridge until ready to serve.
For the salsa, mix together all the ingredients and set aside.
When ready to serve, cut each celeriac half into 4 long wedges, each about 4-5cm/1½-2in wide. Place 2 wedges onto each plate and then spoon the horseradish cream on top. Layer the trout or salmon on top of this, followed by the salsa. Drizzle each portion with lemon juice and serve.