Wash, blanch and cut the spinach into pieces (save 80 ml of the blanching water).
Steam the chopped onions in oil until translucent, add the spinach and season with the vegetable stock. Add the cream and blanching water and squeeze the garlic.
Drizzle the fish fillets with lemon juice and lightly salt.
Put half of the spinach in a baking dish, place the fish fillets on top and cover with the remaining spinach.
Cut the cheese into thin slices and place on top of the vegetables.