Heat oven to 200C/180C fan/gas 6. Line a roasting tin with foil and put the trout fillets in it, skin-side down. Rub with a little olive oil, sprinkle over the chilli flakes, season and top with the parsley. Add half the lemon, then cover the tin with foil and roast for 12-15 mins until the fish is opaque and flaking. Remove from the oven and leave to rest.
Put the buckwheat in a pan, cover with 300ml water, add a pinch of salt and a bay leaf and bring to the boil. Skim off any scum, reduce the heat and simmer for 8 mins. Turn off the heat, cover with a lid or plate, and leave to puff up for another 10 mins.
Put the red onion in a large bowl with a pinch of salt and squeeze over the remaining lemon. Bring a pan of salted water to the boil and blanch the asparagus for 2-3 mins until a knife goes in easily. Drain and set aside.
Once the buckwheat has absorbed the water, tip into a sieve and rinse. While it’s still warm, tip into the bowl with the onion, and add the herbs and watercress. Flake in the trout, then squeeze over the juice from the roasted lemon half and add 2 tbsp of the olive oil and season. Whisk the remaining olive oil with the crème fraîche, honey and more seasoning. Divide between plates, drizzle with the dressing and top with the asparagus and toasted pumpkin seeds.