Melt the butter in a frying pan over medium heat. Add leek and apple. Season lightly with salt. Fry the leek and apple until the leek is soft and starts to color slightly. Set aside and keep warm.
In the meantime, rub the fillets with some garam masala. Heat the oil in a large frying pan over medium heat. Fry the fillets gently so the coating darkens and the fish is barely cooked. Set aside and keep warm.
Also in the meantime, place a tablespoon or two of water in a small saucepan and bring to a boil. Whisk in the chilled butter. Season with the piment d'Espelette. Foam the sauce with a stick blender.
Divide the leek and apple mixture between heated serving plates. Arrange the fish fillets on top. Spoon the sauce over the fish. Sprinkle the chives over the whole dish. Serve immediately.