Cook the spinach in boiling water. Drain and squeeze. Cook them lightly for 2 minutes in a soup spoon of butter, then put the spinach into an oven dish.

Melt a soup spoon of butter in a saucepan, and lightly cook the, onions and the filets on both sides. Add the white wine, salt and water and let cook 8 to 10 minutes on a very low heat. After this, carefully place the fillets on the spinach. Reduce the cooking sauce by one half. Add the Mornay sauce and mix. Let thicken, then pour over the fish. Sprinkle with grated gruyere and brown for 5 minutes in the oven.