Julienne the carrots, leek and celery and sauté in butter. Wash the spinach and let it collapse dripping wet in butter. Mix with the vegetables and season with salt and pepper.
Put the vegetables in an ovenproof dish. Pepper and salt the fish fillets and sauté briefly in butter, then place on the vegetable mixture and sprinkle with crème fraîche and Gruyère.
Gratinate in the oven at 200 degrees for about 15 minutes.