For the salsa, heat the olive oil over medium heat in a large frying pan. Add the onion and saute for 1-2 minutes. Add tomato, apple, cucumber, thyme and half the chervil. Cook for 1 minute. Remove from heat and transfer salsa mixture to a bowl. Spoon over the lime juice and set aside.
Return the frying pan to the heat, add a little olive oil and pan-fry the fillets until slightly golden, about 2 minutes each side. To serve, place the fillets on serving plates and top with salsa and the remaining chervil.