Place new potatoes in a pot with enough water to cover, and bring to a boil. Cook 10 minutes or until tender.
Melt the butter in a frypan over medium heat and saute the mushrooms and onion until tender. Sprinkle with lemon juice and season with sea salt and cracked black pepper.
Slash the perch fillets on both sides and insert the chervil. Rub with chilli oil, sea salt and cracked black pepper. Place fillets on a baking tray and grill 5 minutes on each side, or until easily flaked with a fork. Drizzle with pesto and serve over new potatoes, mushrooms and onion.