In a small bowl, whisk yoghurt, mustard and lemon juice and set aside.
Boil potatoes in salted water for 15-20 minutes until tender. Drain and lightly drizzle with olive oil, then return them to the pot and cover with a tea towel to keep warm.
Remove woody ends of asparagus spears, place them in boiling water, cover and cook for three to five minutes, depending on thickness. Remove, drain and serve right away, or plunge spears into ice-cold water to serve cold.
Season the trout generously with salt and pepper, before massaging with 1/2 tbsp of olive oil, coating evenly. Heat a frying pan over medium-high heat. Place fillets skin-side down and cook for 2-3 minutes until skin is crisp. Carefully flip and cook for an additional minute or so, depending on thickness, or turn off heat immediately - the residual heat will continue to cook the fillets.
To assemble the dish: Toss together the potatoes, asparagus, capers, spring onions, seasoning and olive oil. Arrange the mixture on plates. Top with the trout fillets and drizzle the yogurt mustard dressing over them. Garnish with dill, chervil, rocket, or pea shoots as desired.